Add a checkbox to allow administrators to re-send the welcome email to users.

Summary:
See title - This simply adds a checkbox to the "Edit User" page in the
admin view, to allow an administrator to re-send the "Welcome to Phabricator"
email.

Test Plan:
Sent myself another welcome email using the checkbox.
Created a new user using the admin panel, to make sure emails still get
sent for new users.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1524

Differential Revision: https://secure.phabricator.com/D3081