POSSE post discovery to support link-less syndicated content

The first time a syndicated post is encountered, fetch the author's homepage. 
For each h-entry u-url, if it has not been seen on a previous iteration, fetch the
permalink and check for rel=syndication/u-syndication links. Store any discovered
relationships to support future comment backfeed.

See http://indiewebcamp.com/posse-post-discovery#Discover_POSSE_copies_via_rel-syndication 
for more detail.

Remaining questions: should we limit the number of h-entries that we'll search 
for a given h-feed? I'm currently storing relationships indefinitely in the NDB, 
should they be refetched every so often?

Possible todo: add a cron job to poll author's h-feeds once a day or so. 

P.S. Sorry that it's kind of a huge PR...This felt like the smallest reasonable incremental chunk.