Make the feed framework a lot more flexible and give the possibility to have
a feed for each architecture. You can drill down even more than also get a
feed for a particular repo; some might find this helpful for something like
tracking [testing]. Implements FS#12939.

I also bumped up the number of items available in each of these feeds; since
it is full of a bunch of small items it might be more helpful to have more
available and it should also prevent fewer ones from being missed.

The UI isn't exactly spectacular, but I figured some sort of page is better
than none listing all the various feeds you can pull from.

Signed-off-by: Dan McGee <dan@archlinux.org>