One more try to restore previous Atom <id>'s

Original behavior is http://zotero.org/:username/items/:key for user
items, http://zotero.org/groups/:slug/items/:key for public group items,
and http://zotero.org/groups/:id/items/:key for private groups

This is set to change in v3, though that's up for debate, since people
with existing feed subscriptions probably don't monitor the dev list.