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.

This engine was replaced with CommonMarker in 11.4, it was deprecated
since then.

Signed-off-by: Tao Wang <twang2218@gmail.com>
Signed-off-by: Rémy Coutable <remy@rymai.me>

This is to DRY the repeated file size check.

Move spec and constants to Highlight

This replaces the repository param.
This allows more flexiblity as sometimes we have highlight content
not related to repository. Sometimes we know ahead of time the language
of the content. Lastly language determination seems better fit as a
logic in the Blob class.
`repository` param is only used to determine the language, which seems
to be the responsiblity of Blob.

Whenever we already deal with a User object, let's use the more specific
method avatar_icon_for_user.

Recudes the number of queries within the snippet search from approx. 50
to approx. 14 by preloading the authors

Part of #27392

Also enabled syntaax highlight on the search results

Closes #26832

Instead of rendering this value server-side, we use Javascript and Gon
to apply the user's color scheme (or the default) to any syntax
highlighted code blocks.

This will make it easier to cache these blocks in the future because
they're no longer state-dependent.

http://feedback.gitlab.com/forums/176466-general/suggestions/5529795-search-though-snippets