Add user_color_scheme class to file_highlight

Prevents an unstyled flash that occurs when we were only applying the
syntax highlighting class in Javascript. Now for these uncached blobs
the server can add the syntax class, and for (possibly) cached fenced
code blocks, the Javascript will add it as needed.

app/views/shared/_file_highlight.html.haml

-.file-content.code.js-syntax-highlight
+.file-content.code.js-syntax-highlight{ class: user_color_scheme }
   .line-numbers
     - if blob.data.present?
       - blob.data.lines.each_index do |index|