posse-post-discovery: standardize facebook urls

Added special case handling for Facebook sources, to replace
username with numeric user id in syndication urls, when storing
relationships. Since the incoming activities standardize on
numeric id, this gives us a better chance of finding a match.

Addresses #184. Instead of querying by both username and 
user id, I just replace username with user id and always store
one SyndicatedPost. Since we can count on Bridgy
using the user id everywhere internally, I think it's only the
incoming u-syndication links that need to be canonicalized.
Please let me know if I'm oversimplifying the problem!