WIP: Group handling

[fabianfreyer]

• Implement administrative user profiles
  • Add user edit/delete links to user profile
• Implement endpoints to join/remove a user to/from a group
  • Disable if user is last user in the group
• Add group pages that list members
  • Discussion: should this also be the edit form (after all, it's really just a single field, description)
  • Disable deletion of admin group (since hardcoded), add a note to the page.
  • Add member functionality (autocomplete textbox? selecting from a list would become tiresome since the users list becomes pretty long)
• Add a group overview page
• Create group form 

[JanLuca]

LGTM

[fabianfreyer]

ad2becf has not changed, only rebased on current master