Optimized commit diff views, now showing file size and file mode changes

Created by: koenpunt

The following things are in my PR

• Replaced commit status images (diff_file_*.png) with fontawesome icons
• Updated headers of diffs
  • Added filemode changes, although the mode_a of Grit::Diff doesn't return a value and filemode only changes are not available in the diffs array.
  • Added link to file a a button
• Added filesize below images in diff view
• Added overall scss variable $monospace for monospaced font-family (Menlo, Liberation Mono, Consolas, Courier New, andale mono, lucida console, monospace)

Screenshot below: