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