Updated count on merge request badge in navigation

What does this MR do and why?

This updates the count for the merge request button in the navigation bar when the merge_request_dashboard feature flag is enabled. When the merge_request_dashboard feature flag is enabled the count includes merge requests that are waiting for my review, and merge requests I am an assignee of that a reviewer has requested changes.

NOTE This is an experimental feature and will most likely change in the future

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

• Enable merge_request_dashboard
• The merge request badge in the navigation should contain a count of merge requests you are an assignee on and a reviewer has requested changes and merge requests you are reviewer of but haven't reviewed yet.