In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15475/diffs, a
significant amount of work went into eliminating N+1 queries in the
/api/groups/:id/projects endpoint. We can reuse the
`Entities::Project.prepare_relation` call on the projects.

In a group with 2,573 projects on GitLab.com, this change significantly
improves performance:

* 18019 SQL queries down to 21
* Time spent in DB: 70 s down to 384 ms

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49845