R1:2c713b2d25fd

Add "Auth Messages" to support customizing onboarding/welcome flows

Summary:
Ref T13222. Long ago, we had a Config option (`welcome.html`) to let you dump HTML onto the login screen, but this was relatively hard to use and not good from a security perspective.

In some cases this was obsoleted by Dashboards, but there's at least some remaining set of use cases for actual login instructions on the login screen. For example, WMF has some guidance on //which// SSO mechanism to use based on what types of…