Add ContentRemovalRequest viewset

[bjester]

Depends on: #10365

Background

In order to provide support for managing a user's 'My Downloads', we need a new API viewset that will allow us manage removal of content.

Deliverables

The ContentRemovalRequestViewSet

a) It should inherit ValuesViewset
b) It should operate upon the model ContentRemovalRequest

The ContentRemovalRequestViewSet: request creation

a) It should allow creation of a ContentRemovalRequest model for a user
b) It should create the model with the appropriate fields. See ContentRequest.build_for_user

Acceptance criteria

Appropriate tests for the viewset

References

#10137

[rtibbles]

This has been folded into #10379 in order to give a more simplified frontend API interaction experience.