Create daily archive pages

This stopped working when the Index object became a class variable on
Feed. The archive location was set in __init__ so the day never changed.

Now, set the archive location in Index.write_archive so it updates on
each run (and changes with the day).
Merge branch 'hotfix-create-archive-pages'
Use score (not age) when trimming Feed.updates

Previously, a feed update would be dropped if it was 501st entry in
the deque no matter where it actually landed on the index page.

With this commit, sort Feed.updates and drop the 501st update after
it's been sorted by Index.factor_update.