package branch feeds missing

http://feeds.edge.launchpad.net/launchpad-project/revisions.atom (and branches.atom) exist for project groups and projects.

We similarly need them for distros and distroseries.

This is for helping make UDD get wider adoption:

11:06 < lifeless> slangasek: how important is this
11:07 < slangasek> lifeless: I think having a flexible mirroring tool for UDD is a tipping point for its wider adoption; not necessarily asking you to dedicate resources to solving it right at the moment

(the rss feed is an obvious thing to use to control on-demand triggering of mirrors of individual branches).