posse-post-discovery: periodically refetch h-feed

- look for updated rel=syndication links and re-propagate
  Responses that did not have the syndication url the first
  time it was propagated.

- narrowing the scope of the posse-post-discovery transaction.
  the new refetch behavior made this much more imperative.

- added 3 unit tests, one to original_post_discovery_test
  that exercises refetch(), and 2 to tasks_test to test both
  the situation where we should refetch and the situation where
  we shouldn't.

this is intended to address issues #171 and #131
let's see...

...actually i think this is fine as is. if we change `status` to `'new'`, the propagate task will [notice and short circuit out without modifying the Response entity](https://github.com/snarfed/bridgy/tree/master/tasks.py#L446). we'll reprocess the initial post URLs and send duplicate webmentions, but that's fine.

(...i'm now actually double checking this, because i think the current code returns a 500 but still does incorrectly modify the `Response` entity. separate issue though. i'm on it.)