parse_urls does not insert rel="nofollow"

[hellekin]

The parse_urls function is used in output/longtext (e.g. embed plugin and WYSIWYG editors), which opens a huge hole for spammers to get in and post their links.

Blog spam is one of the most important attacks these days, and ensuring all external links add rel="nofollow" would help preventing it.

See related #3369 and #3370.

One way of doing it would be for use elgg_view("output/url") in the callback. And simplify the horrible regular expression...