Rename `markdown_preview` routes to `preview_markdown`
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3461/diffs#note_4584269
These are actions, so the verb should come first.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3461/diffs#note_4584269
These are actions, so the verb should come first.