tweak RSS elements for W3C compatibility

The W3C's RSS feed validator had some things to complain about after [my last round of RSS tweaks](https://github.com/discourse/discourse/pull/1738):
* invalid email in the `<author>` elements
* relative URLs in the `<description>`
* invalid tags and attributes in the `<description>`

(Here are its lists of complaints for a [category feed](http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fmeta.discourse.org%2Fcategory%2Ffeature.rss) and a [topic feed](http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fmeta.discourse.org%2Ft%2Frss-support-for-categories-topics%2F2075.rss).)

I "fixed" the invalid email thing by pulling the `contact_email` admin setting if it's been set. (If it's not set, the installation will always produce invalid feeds; I'm not sure what the best solution for that is, since I don't think it's appropriate to expose the email addresses of actual users.)

Relative URLs are made absolute by using a (newly named) route helper. (Out of curiosity, why is the route name `user` reserved just for the `users#destroy` action?)

As for the invalid tags and attributes: it complains about the `data-foo` attributes on the `<aside>` created when you quote a post, and complains about the `<iframe>`s of youtube embeds. However, everything in the `<description>` is wrapped in a CDATA, so I'm not sure what it's so upset about...