Fixed two bugs:
- Bug #55: Added a default rel_type when adding a user to a community. dotlrn_club::add_user and dotlrn_class::add_user both had it, dotlrn_community::add_user didn't which caused join of a subgroup to break.
- The join link referenced a join.gif image, but this isn't included in the dotlrn package, so I changed it to be words.