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

SyndicatedPost.query_by_original and query_by_syndication now
return lists instead of individual items

Note that this doesn't change the way refetch works -- we won't refetch a post that we already have a relation for to see if it's relationship has changed.

@snarfed, this is feeling like more than I can get my head around right now. I'm confident that I added good tests but not totally confident in the logic. I wanted to go ahead and send to you if you want to review, but also feel free to hold off for a bit.

ref: #222 #259
minor: feel free to use util.trim_nulls if you want. (i'm fine either way.)