Remove N+1 queries with /projects/:project_id/{access_requests,members} API endpoints

We can simplify the code quite a bit and improve performance by using grape-entity merge fields:

https://github.com/ruby-grape/grape-entity/tree/v0.6.0#merge-fields

This performance issue was fixed in https://gitlab.com/gitlab-org/gitlab-ce/commit/5010be7766d08a9adee51c7d16ba71c19ff7dede, but then reverted in https://gitlab.com/gitlab-org/gitlab-ce/commit/d8dd1c1940c929eab324951e3c302d197c5f0dda.

Relates to #42030 (moved)