GitLab email diff improvements

ZD: https://gitlab.com/gitlab-org/gitlab-ce/issues/12820

List of Suggestions:

  1. Completely highlighted modified lines from edge to edge. Makes them more obvious if the line itself is short.
  2. Line numbers.

Also referencing other issues:

  1. Highlight the changed text within the line (Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/3087)

Other Observations

Don't like the "diff language syntax", i.e. the headers giving the lines changed and the +/- notations, however they we do like seeing the function names which are coming as part of that diff language syntax. The git-commit-notifier diffs are more readable.