OStatus: fix remote groups to work with new user_groups/local_groups split.

OStatus: fix remote groups to work with new user_groups/local_groups split.
- fix <activity:subject> generation so we get the profile info (what's available so far)
- use id instead of nickname for group join/leave forms so we can join/leave remote groups
  while the rest of the groups UI remains limited to local groups
  (plugins are responsible for making sure remote notifications and permission checks are done)
- fix remote notification when joining group through OStatus's remote subscribe form