#101 Team sharing form

* adds team sharing field to share form
* splits sharing logic into user based and teams based
* adds TeamAdmin for admin backend
* adds validity check on Team name -> only unused names shall be valid

api/views/views.py

@@ -292,7 +292,7 @@ class AbstractModelShareAPIView(AbstractAPIView):
                 id__in=obj.shared_users
             )
             new_users_objs = obj.shared_users.union(new_users_to_be_added)
-        obj.share_with_list(new_users_objs)
+        obj.share_with_user_list(new_users_objs)
         return True