R1:ab7aceaabf49

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…