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 account you have. On secure, users assume they can register by clicking "Log In With GitHub" or whatever, and it might reduce frustration to tell them upfront that registration is closed.

Some other types of auth messaging could also either use customization or defaults (e.g., the invite/welcome/approve mail).

We could do this with a bunch of Config options, but I'd generally like to move to a world where there's less stuff in Config and more configuration is contextual. I think it tends to be easier to use, and we get a lot of fringe benefits (granular permissions, API, normal transaction logs, more abililty to customize workflows and provide contextual help/hints, etc). Here, for example, we can provide a remarkup preview, which would be trickier with Config.

This does not actually do anything yet.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19992