Configurable trim_feed handling

Introduce additional flags in settings.py for controlling how
feed-trimming (purging old articles) behaves.

* settings.py: There are several new flags:
  + TRIM_STORIES is the master enabled/disabled switch. Enabled by
     default.
  + TRIM_STORIES_KEEP_DAYS is the minimum # of days to keep articles
    around for.
  + TRIM_STORIES_KEEP_NUM is the minimum # of articles to keep per
    feed.

* apps/rss_feeds/models.py (trim_feed): Respect new TRIM_STORIES*
  flags. If TRIM_STORIES is enabled, if there are over
  TRIM_STORIES_KEEP_NUM stories in a given feed, pick a
  threshold-date for the purge by picking the lesser of the two
  dates based on the date of the TRIM_STORIES_KEEP_NUM'th story in
  the feed versus the Now-TRIM_STORIES_KEEP_DAYS date. In other
  words, ensure that we keep at least KEEP_DAYS worth of articles
  and at least KEEP_NUM worth of articles.