CLC-4163 Revoke Google token when disconnecting

https://jira.ets.berkeley.edu/jira/browse/CLC-4163

Includes bonus refactoring: AuthController was a pointless remnant of the era when Canvas was also done through a user-granted OAuth token. Inlined AuthController into GoogleAuthController. 

This area is undercovered by unit tests, which are really hard to write for the OAuth context. It would be really hard to write a UI-based test for it, too.