Xss escaping fixes

Here are some fairly urgent pretty simple fixes to a couple of issues with the recent addition of the rails_xss gem (and all the associated automatic escaping).

It fixes the display of highlighted bits in all the search and fixes the atom feeds too.

I replaced the atom feed with an erb template because the whole of the xml output was being escaped. Rather than try to dive into the xml builder and try to figure out what's going on it seemed simpler to replace the whole thing with a simple erb template.

Once we've moved to Rails 3 we could potentially switch back to using the atom_builder. I'm assuming the escaping issue won't be there then.