Updated reviewers sidebar widget

What does this MR do and why?

Added a review summary to the reviewer sidebar widget that has an assign button that will open the approval rules drawer. The edit button will now open a quick assign dropdown.

Added a button to the merge request widget that will open the approvals drawer component.

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 reviewer_assign_drawer feature flag
• Visit a merge request that has approval rules setup