Add rel="nofollow" to user-submitted links

These patches add rel="nofollow" to `<a>` tags generated from user-submitted content.  This protection covers post bodies and signatures.

Spammers sometimes post links to their domains in order to inflate their search engine ranking.  The rel="nofollow" tag tells search engines not to weigh the link in PageRank calculations.  Therefore it dissuades spammers from posting.