* adds mail sending logic for new notification setting
* adds new templates for user and team based sending
* enhances all email template layout
* adds translations
* adds restorable delete functionality to Team model
* refactors minor code model parts by introducing DeletableObjectMixin
* only non-deleted Teams can be chosen for sharing
* deleted Teams can be restored using the proper function on the backend admin
* deleted Teams do not provide
* adds migration
* 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
