R1:cdd5e3f7ddac

Initialize `$assign_phid` properly in the "!assign" email action

Summary: If you `!assign cahd` when you meant to `!assign chad`, we'll hit an "Undefined variable: assign_phid" a little further down.

Test Plan: Eyeballed it. See IRC.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14291