R1:e55522cade3f

Implement "auth.logout" Conduit API method

Summary:
Ref T7303. Ref T7673. This implements an "auth.logout" which:

- terminates all web sessions;
- terminates the current OAuth token if called via OAuth; and
- may always be called via OAuth.

(Since it consumes an OAuth token, even a "malicious" OAuth application can't really be that much of a jerk with this: it can't continuously log you out, since calling the method once kills the token. The application would need to ask your permission again to…