Add a rough "!history" email command to get an entire object history via email

Summary:
See PHI505. Ref T13124. If you're an agent of a hostile state trying to exfiltrate corporate secrets, you might find yourself foiled if Phabricator is secured behind a VPN.

To assist users in this situation, provide a "!history" command which will dump the entire history of an object in a nice text format and get through the troublesome VPN.

Some issues with this:

• You currently get all the "X added a comment." up top, and then all the comments below. This isn't terribly useful.
• This goes through the "Must Encrypt" flag, but possibly should not? (On the other hand, this is a pretty willful way to bypass it the flag.)

Test Plan: Used bin/mail receive-test ... to send !history commands, got somewhat-useful response mail.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13124

Differential Revision: https://secure.phabricator.com/D19372