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.
aww, give yourself more credit! i understand what you mean, and i definitely appreciate the concern, but given the test coverage and usage in the wild so far, i wouldn't call this an experimental hack any more. i'm basically as confident in it now as in the rest of the codebase. there may be bugs, sure, but all code has bugs. :P

plus, the `Source.canonicalize_syndication_url` method would be _very_ low risk and non-invasive.

i don't feel strongly though. up to you! if you keep it like this, please just add a TODO to make this change later.