Before, the styling for `img:not(.emoji)`` was repeated between
`.md:not(.use-csslab):not(.wiki)` (for comments and the Markdown
preview), `.documentation.wiki` (for help pages), and
`.issuable-details .description .wiki` (for MR and issue descriptions).

In these places, we now simply use `.md`. Wherever we use `.md` but
don't want this image styling (like wikis and blobs), `.md-file` is
added.