Add MR-wide comment as part of a merge request review

Implementation issue for 🎨 Design: Add comment and approval as part of c... (#350394 - closed)

Please refer to epic for more information

Note This issue is marked as blocking Allow "submit review" in combination with addin... (#10869 - closed) as it needs to be implemented first (or they can be developed simultaneously)

Original description

## Problem

Merge request reviews allows to batch multiple comments from the diff, however, users are currently unable to add an MR-wide comment to the review as part of the batch.

Proposal

Allow for general comments to be added from the diff view that do not address a particular portion of code and then go to be displayed on the MR overview.

Customers

https://gitlab.my.salesforce.com/0016100001F2Gsw