Key articles on their title and time.

Links are not unique to an article, because some feeds use their main
page link for whatever their first article is. When their first
article becomes old, they change the link field to point to the
archive.

Fixes #96
Set LastFetch time for a feed even when we fail to read it from its source.

Fixes #99
Make the root page show latest articles.

These are articles that were published within the last 18 hours. This
both makes it easier to find new articles, since fewer are shown, and
should reduce the datastore reads, since fewer articles are read.

Fixes #47