disable emailing on more publish errors
add paging to /users page. fixes #167

uses naive offset instead of cursor(s). see UsersHandler docstring for details.
posse-post-discovery: periodically refetch h-feed

- look for updated rel=syndication links and re-propagate
  Responses that did not have the syndication url the first
  time it was propagated.

- narrowing the scope of the posse-post-discovery transaction

- added 3 unit tests, one to original_post_discovery_test
  that exercises refetch(), and 2 to tasks_test to test both
  the situation where we should refetch and the situation where
  we shouldn't.
posse-post-discovery: refactoring and cleanup
unit tests for webmention-tools discovering relative webmention endpoint URLs

implemented for Link HTTP header in vrypan/webmention-tools@52eda77c5cbd892ae8c6bcd03ed62c327ddbb88c
posse-post-discovery: append to unsent list

was incorrectly replacing the unsent list, but tasks might not
be complete yet.
posse-post-discovery: SyndicatedPosts unit tests
posse-post-discovery: ensure last_syndication_url is set
Merge pull request #192 from kylewm/ppd-update-syndication-urls

posse-post-discovery: periodically refetch h-feed
don't email me about #177
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']
temporarily add tommorris.org to webmention blacklist (its handler is broken)
new FAQs: how far back polling looks, how to construct source URLs
add /post/... to source URL format section
add mf2 handlers to robots.txt
source URLs support ?format=json
fix bug #196 in handlers that omitted mentions when there was a rel-syndication link