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.