Show advertising message for initial install (or updated) to show sidebar panel manually

[staktrace]

Short description

After upgrading to WebExtension version of TST, the sidebar doesn't automatically show. Filing this as a result of discussion on https://bugzilla.mozilla.org/show_bug.cgi?id=1403183

Steps to reproduce

1. Start Firefox (version < 57) with clean profile.
2. Install TST (non-webextension version)
3. Update Firefox to 57. This should also update TST to 2.0 which is the WebExtension version

Expected result

When the WebExt version of TST is installed, the sidebar panel is made visible.

Actual result

It is not made visible. This makes it feel like TST is broken.

I originally filed a bug against Mozilla for this, https://bugzilla.mozilla.org/show_bug.cgi?id=1403183 but it's not considered a bug by the add-ons team. Instead the suggested fix is that TST open a new tab advising the user to make the sidebar visible.

[piroor]

Because it is not allowed that WebExtensions addon opens the sidebar automatically without any user action, I think it is the last solution left for us: showing announcement message at initial startup of TST 2.0.

[piroor]

The API: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/runtime/OnInstalledReason

[piroor]

Oops, I wrongly mark the commit is related to this... It should be #1038

[piroor]

By recent commits, now a message tab is shown automatically when TST is updated from legacy to 2.0.

[staktrace]

Thanks!