R1:f12af0383665

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…