Maniphest - use subscribers framework properly

Summary: Fixes T5604. This should fix some random bugs, lets us move forward more easily, and all that good stuff about killing code debt.

Test Plan:

• Conduit method maniphest.createtask
  • verified creating user subscribed
  • verified subscription transaction
• Conduit method maniphest.update
  • verified subscribers set as specified to ccPHIDs parameter
  • verified subscription transaction
• Herald
  • verified herald rule to add subscriber worked
  • verified no subscribers removed accidentally
• edit controller
  • test create and verify author gets added IFF they put themselves in subscribers control box
  • test update gets set to exactly what user enters
• lipsum generator'd tasks work
• bulk add subscribers works
• bulk remove subscriber works
• detail controller
  • added myself by leaving a comment
  • added another user via explicit action
  • added another user via implicit mention
• task merge via search attach controller
• mail reply handler
  • add subscriber via ./bin/mail receive-test
  • unsubscribe via ./bin/mail receive-test

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5604

Differential Revision: https://secure.phabricator.com/D10965