Add filter for approved status

What does this MR do and why?

This adds the possibility to filter the MR list page by their approval state.

List filter for merge requests that have not re... (#3159)

Screenshots or screen recordings

No filter	Filter yes	Filter no

How to set up and validate locally

1. Search for a project which has MRs with and without approvals
2. Apply the Approved filter on the MR list for that project

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.