posse-post-discovery: prevent duplicate blank entries

* Fix a logical mistake in original_post_discovery._process_entry that
  allowed multiple duplicate blanks entries for a given original post
  when `refetch`ing a user's feed.
* Another bug in SyndicatedPost.get_or_insert... that would have allowed
  duplicate `syndication-url -> None` entries though this hasn't happened
  yet.

This relates to issue #259