the url to dictate arbitrary static method calls.
* Each xxx_rss helper has a single feed() call which takes an id as the argument
* xxx_rss::available_feedS() only returns feeds when they're applicable (ie
if you're viewing a tag, it won't show you an item feed).
* Feed urls are now in the module/feed_id form so that we can bind a
feed id to a given module
* Tightened up the Rss_Controller by using url::merge and some other tricks.
* Made the slideshow module express its own feed.
replace with a type field with one of two values (head and block). We need to
do this to determine what fields go in the rss block so we can ignore the
definitions that are related to the page head when creating the rss block that
goes into the sidebar.
