Serve SharePoint documents from generic view

[oliverjwroberts]

Description of change

Users wish to see all files/documents relating to a company under a new "files" tab on the company overview page. The caveat is this collection list should show files of various types: those in SharePoint, those uploaded to an S3 bucket, and maybe others in the future.

See CLS2-1300, CLS2-1301, and CLS2-1304 for more information.

This PR adds a model for a SharePointDocument (as we are focusing on this type for the MVP), as well as a GenericDocument which has generic relations to the type-specific document model, and the related object, which in this instance is a Company. However, this may be extended to InvestmentProject's, or other models, in the future.

It also adds a serializer and viewset to enable the retrieval of one or multiple GenericDocument instances, along with the ability to sort the results on the created_on field, and filter them by realted_object_id.

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.65%. Comparing base (c0d85af) to head (787cf42).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5989      +/-   ##
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+ Coverage   96.64%   96.65%   +0.01%     
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  Files        1085     1089       +4     
  Lines       25486    25580      +94     
  Branches     1687     1690       +3     
==========================================
+ Hits        24630    24724      +94     
  Misses        699      699              
  Partials      157      157              

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[DeanElliott96]

Looks good, small suggestion to move test utility functions into the tests folders.