Fix some bot issues

Summary:
  - Deprecate differentialnotification in favor of feednotification; it's strictly better.
  - Fix feed notification + channels.
  - Fix rendering of new-style stories (pholio, macro), which currently fatal.

Test Plan: See chatlog.

Reviewers: codeblock, indiefan

Reviewed By: codeblock

CC: aran

Differential Revision: https://secure.phabricator.com/D4981