Track key authentication metrics

Track key authentication metrics

Logs a 'login' event for logins via Special:UserLogin
and API action=login. Does not log for implicit login after
account creation and for autologin (e.g. based on an active
CentralAuth global login).

Logs an 'accountcreation' event for account creation via
Special:UserLogin/signup and API action=createaccount. Does not
log for autocreation.

Both successful and unsuccessful attempts are logged, except for
failures that throw exceptions (internal errors + some permission
errors).

Bug: T91701
Change-Id: I101b11d05400b073065da10f1e537412309d9102