Backend session expiry

[ryjen]

Describe the bug
Authentication with backends provides a session token that may expire after a certain time for security.

There is no current method of reauthenticating other than deleting and re-adding.

This is a common use-case amongst authentication services that generally have an API to call to 'refresh' a token.

To Reproduce
Steps to reproduce the behavior:

1. Add a backend
2. Wait a few days/weeks
3. Use backend
4. See error

Expected behavior
The application should refresh the session automatically, or the user should be prompted to add their credentials again.

Screenshots

Environment (please complete the following information):

• OS version: Any
• Device: Any
• App Version: 3.1.0-alpha2

Additional context
Any user error message strings need to be localized.

[tladesignz]

Actually, I'm confused, that this error happens at all. Resp. I'm confused about if this error actually means what we think it means.
One would expect that the Dropbox library would handle all of this on its own.

Maybe there's just some func in the Android library we need to call in this case? Or, like every time the user starts the app to make sure access tokens are refreshed?

As a remedy the user can do, we should probably provide a "Reauthenticate" button in the Dropbox settings scene, which currently just shows the user ID and which triggers the authentication process again.

Maybe, we can even ask the Dropbox lib, if the session is good or broken. Only in the latter case, we would show the button.

If there's a function like that in the Dropbox lib, we could also check before a new upload starts.