Use the customizable "Welcome Mail" message in welcome mail

Summary:
Depends on D19994. See PHI1027. If an install has customized the "Welcome Mail" message, include it in welcome mail. A special custom message from the profile screen overrides it, if provided.

(I fiddled with putting the custom message as "placeholder" text in the remarkup area as a hint, but newlines in "placeholder" text appear to have issues in Safari and Firefox. I think this is probably reasonably clear as-is.)

Make both render remarkup-into-text so things like links work properly, as it's reasonably likely that installs will want to link to things.

Test Plan:

• With custom "Welcome Mail" text, sent mail with no custom override (got custom text) and a custom override (got overridden text).
• Linked to some stuff, got sensible links in the mail (bin/mail show-outbound).

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19995