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.
move canonicalize_syndication_url to Source

with special-case handling moved to the individual subclasses
(FacebookPage only at this time). moved from original_post_discovery.
get FacebookPage numeric user id w/ key.id(), not id
Merge pull request #188 from kylewm/ppd-facebook-urls

posse-post-discovery: standardize facebook urls