posse-post-discovery: check Response.activity_json['url']

- when re-processing old Responses, check the top-level url field
  rather than the incorrect activity['object']['url'] field.
- more robust test - multiple sources, simulate common situation
  where instagram photo is PESOSed, then later POSSEd to twitter.
  we expect refetch to catch the updated twitter URL and erase
  the blank
posse-post-discovery: clean up based on review

- remove unnecessary check for response.sent not None
- consolidate s/snarfed.org/localhost checks to
  util.replace_test_domains_with_localhost
- improve previous check of response.activity_json['url'] to fallback on
  activity ['object']['url'] if the former isn't available.
posse-post-discovery: minor tidying of unnecessary local var
Merge pull request #194 from kylewm/ppd-update-syndication-urls

posse-post-discovery: check Response.activity_json['url']