Allow caching of Special:CentralAutoLogin when safe

Special:CentralAutoLogin/start and
Special:CentralAutoLogin/checkLoggedIn are safe to cache, because
they only differ based on the session cookies (which the cache already
varies on) and don't have any side effects.

The other methods in the login flow do need to be uncached, though, as
they do have side effects.

Change-Id: Ie17b277857529f00a659cdbc933ff6c31fb616ed
Merge "Allow caching of Special:CentralAutoLogin when safe"