R1:6bb31de30596

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…