Enable Markdown preview for issues, merge requests, milestones, and notes
Created by: mr-vinn
This implements an accepted suggestion from the GitLab feedback forum
I added the Write/Preview tabs from the note form to other areas so that Markdown previews are now available when creating and editing notes, issues, merge requests, and milestones.
This is similar to #6356, but that PR was closed and the source branch seems to be gone now. @cirosantilli my apologies for duplicating your PR, but I've really been missing this feature in GitLab.
