R1:fe71b34c6883

Add a "refresh" action for external accounts

Summary:
Ref T1536. This is equivalent to logging out and logging back in again, but a bit less disruptive for users. For some providers (like Google), this may eventually do something different (Google has a "force" parameter which forces re-auth and is ostensibly required to refresh long-lived tokens).

Broadly, this process fixes OAuth accounts with busted access tokens so we can do API stuff. For other accounts, it mostly just syncs profile pictures.

Test…