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 Plan:
Refreshed LDAP and Oauth accounts, linked OAuth accounts, hit error conditions.

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Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6290