Endpoints for userlist/learningpath memberships

[mbertrand]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/5159

Description (What does it do?)

• Creates an endpoint to retrieve all learning path relationships
• Creates an endpoint to retrieve all user list relationships

How can this be tested?

• Log in as an admin or a learning path editor and go to http://api.open.odl.local:8063/api/v1/learningpaths/membership/
• You should get a list of all the relationships (id, parent, child attributes) from all the learning paths.
• Create a couple user lists if you haven't already.
• Go to http://api.open.odl.local:8063/api/v1/userlists/membership/
• You should get a list of all the relationships from all of your userlists
• Log in as another user, with no or different user lists, and who is not an admin or learning path editor, then go back to http://api.open.odl.local:8063/api/v1/userlists/membership/
• You should get a list of all the relationships (if any) from all of this user's userlists
• Go to http://api.open.odl.local:8063/api/v1/learningpaths/membership/, you should get a 403
• Log out
• Go back to both urls, you should get a 403

[gumaerc]

👍 LGTM!