urlize abstracts

I've looked through the database and I can't find any accepted
proposals that actually contain anything to linkify - but I've had
several requests from people that I think are reasonable, things like
being able to link to their project (we ask them to provide a project
url but we don't use that link)

This seems like a reasonably fast way to let those people add a
link. We still have to manually accept changes to their proposal so we
have a chance to look at the content of the url if we want.

The urlize filter adds rel=nofollow to the link automatically.