last_checked_at subscription attribute

Previously, the plan had been to use last_build_date to decide
when to check feeds for new entries (taking it from the
lastBuildDate element supplied by some feeds, and
conservatively falling back to DateTime.now when not
supplied). But I think I want more aggressive caching behavior,
so I guess I'll used the last-checked-at time instead (still
using the update frequency period provided by the feed, or
falling back to a default if not supplied)?
handles errors during feed parsing
introduces deletion of user subscriptions
shares Subscriptions between users when applicable

If a feed is already being tracked (presumably by another user),
we should use the existing model and entries, rather than
wastefully refetching that which we already have.
cosmetics

* redesigns navigation header
* license in footer, but no logo
* entry width considers screen size, bylines deemed safe