#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/models/user.py

@@ -160,3 +160,15 @@ class User(AbstractUser):
         else:
             token = self.api_token
         return token
+
+    @property
+    def shared_teams(self):
+        """ Wrapper for fetching active teams of this user
+
+        Returns:
+
+        """
+        shared_teams = self.teams.filter(
+            deleted__isnull=True
+        )
+        return shared_teams