Compare branch/tag page needs some UX love

The compare branch/tag page is a little out of touch, maybe we should give it some love. It isn't responsive yet.

You can view it here: https://gitlab.com/gitlab-org/gitlab-ce/compare?from=master&to=master

Please be aware that gitlab-ce!6550 changes the dropdowns and will be adding some temporary responsive styling to get the MR merged.