posse-post-discovery: Support many-to-many relationships

SyndicatedPost.query_by_original and query_by_syndication now
return lists instead of individual items
posse-post-discovery: cleanup and refactor many-to-many queries

* SyndicatedPost.query_by... methods were not pulling their weight.
  Inlined them in the few places they were being called

* SyndicatedPost.insert() now only handles inserting real (not blank)
  relationships, making the logical much easier to follow/verify

* added SyndicatedPost.insert_...blank utility methods to check for
  pre-existing blanks before inserting new ones.
ppd many-to-many: clean up based on review comments
Merge pull request #261 from kylewm/ppd-many-to-many

posse-post-discovery: Support many-to-many relationships