Draft: Add 'Start a review' button to merge request overview

What does this MR do and why?

This MR adds a 'Start a review' button to the Overview page of a merge request when no draft comments are present.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open any MR
2. Scroll to the editor box
3. Enter some comment
4. Click 'Start a review' button
5. A comment should be added as a draft comment and review bar should appear