Add new authentication-related hooks (and remove one) replacing some AuthPlugin methods

Add new authentication-related hooks (and remove one) replacing some AuthPlugin methods

* LocalUserCreated: Replaces AuthPlugin::initUser()
* UserGroupsChanged: Replaces AuthPlugin::updateExternalDBGroups()
** The similar UserRights hook is deprecated, mainly to get rid of the
   passing of $user by reference.
* UserIsHidden: Replaces AuthPluginUser::isHidden()
* UserIsLocked: Replaces AuthPluginUser::isLocked()
* UserLoggedIn: Replaces AuthPlugin::updateUser()

Also, AuthPlugin::updateExternalDB() is deprecated in favor of the
existing UserSaveSettings hook.

Also, 'ResetSessionID' has been removed. Nothing uses it, I don't know
why I even added it in the first place.

Also, replacing the User object passed to AuthPlugin::initUser() and
AuthPlugin::updateUser() will now raise a warning.

Change-Id: If7474cfb26a29b11c2e78147069419ca3b1cba95