Make phabot never private message anyone.

Summary:
This replaces D4175 and makes it so phabot doesn't message anyone.

The reasons for this are twofold:

- It was possible to get information from the bot, by private messaging it, even
  if the bot was only in a +i channel (on a public network) -- meaning that if
  someone knew the nickname of the bot, they could obtain e.g. ticket names
  or diff titles.
- The other time it messaged people was when you typed e.g. "somenick: T123".
  Most times when this is triggered, it's done so on accident.

See discussion on the old revision (D4175).

Test Plan:
  15:29:33 ::: Irssi: Starting query in quartz with cb-phabot
  15:29:38 <relrod> T2
  (nothing back)

and

  15:29:21 <@relrod> rublets: T1
  15:29:21 < cb-phabot> T1: asdfasdf (Priority: Needs Triage) - http://local.elrod.me/T1

Reviewers: epriestley, btrahan, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4339