allow adding generic comments to a merge request

Description

It is possible to attach a comment to a line of code (e.g. via a merge request). Sometimes during a review we have a general comment that does not apply to a specific line of code / applies to all changes. It would be useful to add such comment without having to add it to a specific line of code.

Proposal

Allow adding generic comments to merge requests (not tied to a specific line of code).