RSS for topics in a category

Creates a new route for category RSS. It raises if you try to get the RSS for "uncategorized".

Things I'm not happy about:
* having to assign @category so the normal show view can get at it to draw the autodiscovery tag
* all the URL nonsense in the RSS view
* that there isn't a feed for "uncategorized" *

\* I wanted there to be one, but it meant a slew of additional URL nonsense in the views.