#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/models/team.py

@@ -93,3 +93,17 @@ class Team(UuidModel):
         """
         mailer = Mailer()
         mailer.send_mail_shared_data_deleted_team(obj_identifier, obj_title, self)
+
+    def remove_user(self, user):
+        """ Removes a user from the team
+
+        Args:
+            user (User): The user to be removed
+
+        Returns:
+
+        """
+        self.users.remove(user)
+        if self.admin == user:
+            self.admin = self.users.first()
+            self.save()