- #2153 remove require for gravatar

We can look at the PushCommits object to see how many commits were
included in a commit, and add some template logic to only show the
comparison link when there are at least 2 commits in a push. We also
correct the link to display the number of commits.

- copy link button: #1396, #1168, #1668,
- synxtax highlight: #1712, #1549, #1315, #670
- z-index: #1942