Improved perceived performance on the new merge request dashboard

What does this MR do and why?

The database query to get the count for the number of merge requests in a list can take much longer than the query to get the merge requests. With this change we request the count separately so that we can load the merge requests quickly and display them without waiting for the count query to finish.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.