#146 Team leave

* 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

user/urls.py

@@ -20,5 +20,6 @@ urlpatterns = [
     path("team/<id>", data_team_view, name="team-data"),
     path("team/<id>/edit", edit_team_view, name="team-edit"),
     path("team/<id>/remove", remove_team_view, name="team-remove"),
+    path("team/<id>/leave", leave_team_view, name="team-leave"),
 
 ]