Re-use issue/MR counts for the pagination system

What does this MR do?

This MR changes the issue and MR index pages so the pagination re-uses the COUNT(*) query used to count rows per state, removing the need for an additional COUNT(*) query. This in turn reduces the page loading times by roughly 30 milliseconds.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• 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

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/27168