Cache the number of open issues and merge requests

What does this MR do?

This MR adds caching mechanisms for the open issues and MR counters displayed in the project navigation (side)bar. This reduces the loading times of all pages displaying this data (basically any project page) by up to 60 milliseconds.

There is a potential exposure risk associated with the implementation which is mentioned in https://gitlab.com/gitlab-org/gitlab-ce/issues/36622 and in the commit message.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by UX
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/36622

cc @rspeicher @briann