Atom feed

see bug #26

Still needs us to:

 - link to the feed from the rest of the site (both human-readable and for autodiscovery)
 - work out the ordering. most-recently-added seems most useful, but requires us to collect that info
 - decide the content of the feed items -- cf. issue #32

Also: this is Atom only, not RSS -- but I think one feed format is enough?
Expose Atom feeds

links in the head of home and event-list pages, plus an icon on the upcoming event pages.

Probably needs more/differently-placed items, but we can think about that later