Update bp_get_group_join_button() to use new capabilities.

[dcavins]

I missed this function when combing for group capabilities. As is, it's a switch based on group type, but could now be a cascade of elseifs checking for capabilities, like:

• If user can leave the group
• If user can receive invites to this group, and has an outstanding invite
• If user can join the group
• If user can request membership