Allow to filter the final message of signup user notification email

[benoitchantre]

When a signup user notification is sent, we cannot filter the final message contained in $message.
We can only filter one part of the message.

This prevent us to send nice formatted emails using HTML (to send a formatted email, we need add include the message inside an HTML template).

[wido]

What about a page setting where it's possible to write a custom message in a tinymce (may be in future with gutenberg) and support for a small (expandible) set of placeholders?

[ideag]

Which message is this referring to? I can't seem to find a non-filterable one. @benoitchantre could you clarify?