Change the names of the ActiveRecord models involved in Design Management

Summary

This is an issue to discuss changing the ActiveRecord model names for the Design Management models.

Created out of a discussion from @alexkalderimis https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15561#note_207277482 where he raised the idea.

Improvements

Model names would more accurately reflect what they represent conceptually, and be easier to reason about when using.

Risks

There's a risk that branches will break when they are rebased with master after a change like this was merged. We should communicate to all developers working on Design Management branches to be aware of any changes that are pending.