Move "Password Reset" email to "PeopleMailEngine"

Summary:
Ref T13343. This makes "Password Reset" email a little more consistent with other modern types of email. My expectation is that this patch has no functional changes, just organizes code a little more consistently.

The new setRecipientAddress() mechanism deals with the case where the user types a secondary (but still verified) address.

Test Plan:

• Sent a normal "login with email" email.
• Sent a "login with email to set password" email by trying to set a password on an account with no password yet.
• Tried to email reset a bot account (no dice: they can't do web logins so this operation isn't valid).
• Tested existing "PeopleMailEngine" subclasses:
  • Created a new user and sent a "welcome" email.
  • Renamed a user and sent a "username changed" email.
• Reviewed all generated mail with bin/mail list-outbound.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13343

Differential Revision: https://secure.phabricator.com/D20662