Submit review should send webhooks

When a reviewer clicks "Submit review" with "Request Changes" selected in the merge request review dialog, no merge request webhook event is fired. This is the remaining half of #517909 (closed) that was not addressed by !205274 (merged).

Steps to reproduce

1. Create a webhook at the group or project level with merge request events enabled
2. Create a merge request and set a reviewer
3. As the reviewer, add a comment to any line of the changes and click "Start a review"
4. Click "Finish review", select "Request changes", and click "Submit review"

What is the current behavior?

• No merge request webhook event is generated
• Only a note (comment) webhook event fires if summary text is included

What is the expected behavior?

A merge request webhook event should be fired with:

• Reviewer state information in the payload
• Similar structure to the approval webhook event