#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/admin.py

@@ -74,6 +74,15 @@ class TeamAdmin(admin.ModelAdmin):
         "name",
         "description",
     ]
+    filter_horizontal = [
+        "users"
+    ]
+
+    def formfield_for_foreignkey(self, db_field, request, **kwargs):
+        if db_field.name == "admin":
+            team_id = request.resolver_match.kwargs.get("object_id", None)
+            kwargs["queryset"] = User.objects.filter(teams__id__in=[team_id])
+        return super().formfield_for_foreignkey(db_field, request, **kwargs)
 
 
 admin.site.register(User, UserAdmin)