Add non-diff comments to reviews

Problem to solve

A review comment does not have to be about code that was written but can also be about code that was not written or general architecture or design. Merge request reviews however don't allow to start new threads.

Intended users

Mostly developers but in general everybody who reviews merge requests.

Further details

It is currently possible to add a review comment to

a line in the diff (starting a new thread)
an existing thread on the diff
an existing thread on the discussion tab (including non-diff threads)

It is not possible to start a new thread on the discussion tab as part of the review.

Proposal

Maybe just add a new Add to review option in the existing dropdown button: