* adds admin column on team index view
* refactors Team model, so multiple members can become admins
* adds team migration for switch from fkey->m2m structure
* renames 'Group' to 'Permission' on user index view to avoid confusion between 'Groups' and Teams
* adds new autocomplete route for team-admin selection based on already selected members of the TeamForm
* adds button and functionality for leaving a team
* if the admin leaves the team, another user will be chosen as new admin automatically
* improves Team (django) admin backend
* better control over user adding-removing
* only added team members are selectable as admin
