#169 Team delete-restore

* 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

user/views.py

@@ -163,7 +163,7 @@ def index_team_view(request: HttpRequest):
     template = "user/team/index.html"
     user = request.user
     context = {
-        "teams": user.teams.all(),
+        "teams": user.shared_teams,
         "tab_title": _("Teams"),
     }
     context = BaseContext(request, context).context