Fix for broken profile flag admin UI: delete stray flag entries when users are delete...

Fix for broken profile flag admin UI: delete stray flag entries when users are deleted so broken entries don't litter the lookups.

* added ProfileDeleteRelated event to match UserDeleteRelated, to allow plugins to add extra related tables on profile deletion
* UserFlagPlugin: deleting flags when target profile is deleted
* UserFlagPlugin: deleting flags when flagging user is deleted
* UserFlagPlugin: fix for autoloader -- class names are case-insensitive. We may get lowercase class names coming in at times, such as when creating DB objects programatically from a table name.

Note that any already-existing bogus entries need to be removed from the database:
select * from user_flag_profile where (select id from profile where id=profile_id) is null;
select * from user_flag_profile where (select id from user where id=user_id) is null;