docs: add login and user mode docs

Author: pyphilia

docs/authentication/authenticaton-proposals.md

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+# User Authentication
+
+Since Graasp Desktop v.0.15.4, a user authentication is available in order for multiple users to use the application.
+
+![Login Screen](./img/loginScreen.png)
+
+The authentication process is triggered right at the launch of the application. It asks for a username before allowing access to the application. Each new username will create a corresponding user account on the fly. If a username already exists in the database, the user will sign in with the corresponding account and be able to access previously stored data.
+
+One can use the application anonymously without restriction, but any saved data will not be retrievable after the user closes the
+session.
+
+In order to automatically authenticate whenever the application is launched, the
+current user session is stored in a dedicated part of the local storage and is used to sign
+in. When the user signs out, the cached session is cleared and the application redirects
+to the login screen.
+
+## Alternative solution: Space-level Authentication (online)
+
+Graasp offers a practical method for students to access spaces. From a link, before accessing a space, students create personal credentials, resulting in a Light account. This account allows to save data for this student for this space only.
+
+If online data from Light users are required in Graasp Desktop, it is necessary to implement a authentication procedure triggered each time a space is visited.
+
+However, as of today's requirements, Graasp Desktop is used in poorly connected countries. Trying to fetch data online is thus not necessary. Additionally, authenticating at a space-level is rather tedious and tends to be repetitive in a context where the user only consumes and visits spaces. Security should also be handled with high precautions.
+
+The following schema depicts a proposed space-level login. Blue steps run while online, whereas grey steps are performed in an offline environment. If online, the application connects to and fetch the online account user data. If offline, the application skips the online procedure and immediately access the space.
+
+![Space-level Authentication Flowchart](./img/spaceLevelAuthentication.png)

docs/authentication/img/loginScreen.png

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+# User Modes
+
+Though different user accounts can be created to access the application and save personal data, spaces are shared among all the users. To handle these spaces (avoid unexpected action on other users), we divide users into different categories/modes/stakeholders, leading to different permissions.
+
+## Stakeholders in Graasp Desktop
+
+### Teachers
+
+Teachers need to manage spaces (as they can online) and their students. Assuming that
+spaces are exclusively created and edited on the main online platform, Graasp Desktop
+is primarily a common pedagogical course support to provide the spaces to their classes
+in an offline context.
+Additionally, teachers use Graasp Desktop to gather and browse their students’ data in
+order to gain insight of their progression throughout the courses, as well as to
+eventually correct their submissions.
+
+### Students
+
+Students use Graasp Desktop to learn in offline environments, either during classes or
+remotely at home. They visit spaces created by their teacher, and might also visit other
+interesting spaces if they have a stable internet connection. Additionally, students
+eventually save resources and leave activity traces while visiting spaces. Such data
+results in interesting analytics of their learning sessions, that students can display to
+evaluate their learning progression. Finally, students share their resources with their
+friends or teachers in order to be evaluated.
+
+### Developers
+
+Even though they are not directly related to the pedagogical purpose of the
+application, developers are necessary in order to maintain and improve the
+functionalities of the Graasp Desktop application. For instance, they need full access to
+the database and to all the application’s features, as well as particular development
+tools to implement new features.
+
+### Researchers
+
+Graasp Desktop is an educational support to collect meaningful data from learners,
+and researchers can use it to conduct their studies. They require specific applications
+to track the users, as well as a mechanism to manage consent from users to have legal
+access to their data.
+While Developers and Researchers also have their importance in the development of
+Graasp Desktop and could have their own modes, as part of this thesis, we only focus
+on teacher and student differences in order to develop a teacher mode.
+
+|           Action            | Student  |             Teacher              |
+| :-------------------------: | :------: | :------------------------------: |
+|        Visit a Space        |   yes    |               yes                |
+|       Export a space        |   yes    |               yes                |
+|        Save a Space         |    no    |               yes                |
+|         Add a Space         |    no    |               yes                |
+|       Delete a space        |    no    |               yes                |
+|        Sync a space         |    no    |               yes                |
+|        Load a space         |    no    |               yes                |
+|      Load space's data      | no/yes\* |               yes                |
+| Classrooms functionalities  |    no    |               yes                |
+| Displayed data in dashboard | own data | own and other users' shared data |
+
+<center>*Student*'s and *Teacher*'s permissions comparison</center>
+
+## Implementation
+
+Since Graasp Desktop 0.15.4, the _Student_ and _Teacher_ modes are available. By default, a user is a _Student_. To become a _Teacher_, the setting **Student Mode** should be disabled in the settings. _Developer_ and _Research_ are not yet available.