Description
I as a user am able to add problems to any other list provided slug and problemID.

To Reproduce
Authenticate yourself.

Send a response to url /lists/userlist/add with POST data

{ "prob_id": "TEST", "slug": "to-do-2505", "platform": "C" } 

Expected behavior
If I am the owner of the list, this problem is successfully added, but if I am not the owner of this list, it should throw an error.

Actual behavior
Problem is added to the list irrespective of the owner. Users can add problems to others lists.

Function Responsible for this - codedigger.lists.views.UserlistAddProblemView