Allow administrators to provide custom welcome text when welcoming users on the profile workflow

Summary:
See PHI1027. Currently, we allow you to customize invite email, but not most other types of email (approve, welcome). As a step forward, also allow welcome email to be customized with a message.

I considered separating the custom text from the main text with something heavyhanded ("alice added this custom message:") or a beautiful ASCII art divider like one of these:

https://www.asciiart.eu/art-and-design/dividers

...but nothing truly sung to me.

This only works on the profile flow for now. I'm planning to let you set a default message. I may or may not let you customize from "Create New User", seems like the default message probably covers most of that. Probably won't touch scripts/user/add_user.php since that's not really exactly super supported.

Test Plan:
Sent mail with and without custom messages, reviewed it with bin/mail show-outbound.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19991