R1:a0c9f9f90c81

Allow installs to customize mail body guidance in the "Email Login" and "Set Password" emails

Summary:
Depends on D20662. Ref T13343. Installs may reasonably want to change the guidance users receive in "Email Login"/"Forgot Password" email.

(In an upcoming change I plan to supply a piece of default guidance, but Auth Messages need a few tweaks for this.)

There's probably little reason to provide guidance on the "Set Password" flow, but any guidance one might issue on the "Email Login" flow probably…