Detect excessive blog fetches

Detect excessive blog fetches

If a single fetch from a blog grabs more than <n> entries (the default
being 4), this usually indicates a misconfigured blog and it will end up
spamming the outgoing feed and twitter, even if later corrected. For
that reason, when this happens, mark all the newly fetched entries as
hidden, and send a notification as such. In the event that they are
actually correct, the user can go in and manually unhide those entries,
which will make them show up again.