Stale authentication

[arturi]

When you connect to Instagram or Google Drive, and then wait for a while, leaving your tab open in the background, for example, authentication times out. But no UI indication for that is shown, so both the list of files for selection, and the files you’ve added are still there — you can select new files and start uploading, but it will error, because authentication is times out.

Proposal: check every X seconds (30?) that we are still authenticated, and update state, so that it will kick you out to the “authenticate” screen. Maybe remove files that are no longer accessible due to authentication, or ask to re-select too?