Squashed commit of the following:

Squashed commit of the following:

commit fb1dfa9e98ded23fb5bdebae6465424a8cb8acd6
Author: Evan Prodromou <evan@status.net>
Date:   Thu Oct 20 10:40:07 2011 -0400

    Use popular notice stream for favorited page

commit e1d409ff738e39061ad35589d546ce9bed456975
Author: Evan Prodromou <evan@status.net>
Date:   Thu Oct 20 10:32:23 2011 -0400

    Use a caching stream for popular notice section

    Instead of a big cached query, we now use a caching notice stream for
    the popular notice section. It uses a single-table query at the
    bottom, then scopes the notices and filters for silenced users. This
    should be much nicer to our database servers.

    Also clears the popular cache when someone favors or disfavors
    something. A nice optimization would be to save the last weights and
    re-calculate them at invalidation time, adding the new notice (or not)
    depending on its own score. That will have to wait for another day,
    though.

commit e9b7ab4c26c95e755adaff53c3957dcfca31c16b
Author: Evan Prodromou <evan@status.net>
Date:   Thu Oct 20 10:31:14 2011 -0400

    Let CachingNoticeStream users skip the ';last' optimization