feat: Editor Table to store which user has access to edit which lists #43

User can add their friends and give them edit access.

This way many students can edit a list and store the problems in that common list.

Editor Model

User: Foreign Key
List: Foreign Key

Routes to add/ modify -

• Owner of the list can add members/ other users to edit a list
• Now added Users have the right to add problems to that list.
  Route : /lists​/userlist​/add bugfix: User is able to add problems to other user list.  #36
• Users can view their own lists and shared lists.
  Route : /lists/user/:username feat: a route to get all lists of a user #38