[Bug 857129] Add diff view to KB revisions list.

To test: go to `/kb/revisions`, click one of the magnifying classes next to a revision. There should be a fragment load of the diff for that revision. Click the close button, and it should slide away.

Some revisions don't have diffs. This is because they were not based on any article.

The bug says that this would require adding a new ajax view and tests for it. InsteadI reused a lot of code from the other diff page, so I didn't have to do this. All the changes here are in templates, JS, and CSS.

r?