Review Board exposes a lot of URLs that search engine crawlers love to

find, many of which don't provide any additional value to search engines

(such as various diff revisions and interdiffs, or alternative views for

a page). These can add a lot of load to the server, as Review Board may

need to repeatedly access repositories and generate diffs.
To help inform crawlers that they don't need to dive into some of these

pages, this change adds some rel="nofollow" attributes to some links.

Tested that the URLs continued to work and didn't appear to break anything.