Cache personal projects count.

What does this MR do?

This changes the User model to cache personal_projects_count for 20 minutes in Redis. The cache is being invalidated on project creation and removal.

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

https://gitlab.com/gitlab-org/gitlab-ce/issues/37462 suggests to pass raw: true to the cache store. I'm not sure why that is, is it necessary?

Is there another way to change ones personal_projects_count (other than creating, deleting, moving a project)?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/37462