posse post discovery: re-propagate old responses the first time we see a domain name

[karadaisy]

this bit @KartikPrabhu

1. user is signed up, but Bridgy doesn't know their domain name ∴ can't do posse-post-discovery
2. Bridgy sees a Response but doesn't find an original post (because no ppd)
3. User re-signs up, Bridgy gets their domain this time
4. User writes a new post and receives a response.
5. Bridgy runs posse-post-discovery for the first time for this Source, it catches the rel=syndication relationship, but doesn't re-propagate the old Response
6. When refetch triggers, it goes back to look for updated rel=syndication links to re-propagate old Responses, but doesn't see this one because it was just discovered by the normal ppd mechanism!

this is the polling log: https://brid-gy.appspot.com/log?start_time=1405657609&;key=aglzfmJyaWQtZ3lyKQsSDkdvb2dsZVBsdXNQYWdlIhUxMTcxMTQwNjA4NTc3MzI0OTY2MjMM

[snarfed]

AUGH SO COMPLIMAKATED :1234: :confounded:

[snarfed]

(kidding! the alg and the bug are both still totally reasonable. if only text on the internet had tone of voice. :P)

[karadaisy]

for reference, I am pretty sure this only applies if Bridgy has never done posse-post-discovery for a particular source (i.e. it has no blank SyndicatedPosts stored in the db). If the account has a URL and changes to a different URL, or adds additional URLs, that shouldn't be a problem.

[snarfed]

@kevinmarks probably would have liked to have this when he added known.kevinmarks.com recently.