[comments] Fixed typo in one of the comments
[models] Added a filter to the joins for Attendee

Added a filter that filters by Event_uid on the joins for the attendee
manululation functions in dbase. This fixed the behavior when there was more than one event, and a user already had a relation between an event already.
[comments] Added comments for Attendee functions

Added the function comments for the Attendee manupulation functions
[models] Remove get_pending_users_for_event

Since the function was not used anywhere in the code it is being removed.