Improve /project API performance

What does this MR do?

It makes the /projects API endpoint faster.

The detailed comparison of the SQL queries are in the comment below.

The time the queries take are measured in gitlab-com/infrastructure#1891. TL;DR:

API query parameter	Old	New
visibility=private	timeout (60+ seconds)	29696.743 ms
membership=true	1830.465 ms	2.015 ms

Are there points in the code the reviewer needs to double check?

Anything isn't broken in ProjectsFinder.

Why was this MR needed?

/v4/projects?visibility=private times out. As reported the SELECT projects.* query is the longest running query for this endpoint, and this MR optimizes that.

Todo

• Write tests for changes in Project model
• Write tests for changes in ProjectFinder
• Print out the affected SQL queries
• Run EXPLAIN ANALYZE to compare results
  • Before refactoring: https://explain.depesz.com/s/oGvX
  • After refactoring: https://explain.depesz.com/s/Ubom
• Check if performance for membership=true query also improved (gitlab-org/gitlab-ce#31937)
• Test timings on staging environment

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#31855 and gitlab-org/gitlab-ce#31937. Might make improvements to gitlab-org/gitlab-ce#24261 too.