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 simpler than we were thinking in the issue discussion, but let me know if I'm missing something.
an alternative would be to add a `canonicalize_syndication_url` method to `Source`. the default implementation would do nothing, and `FacebookPage` would override. thoughts?