Add a bunch of Phacility-specific code to the upstream, thinly veiled as generic code

Summary:
Ref T9304. This adds a "GuidanceEngine" which can generate "Guidance".

In practice, this lets third-party code (rSERVICES) remove and replace instructions in the UI, which is basically only usefulf or us to tell users to go read the documentation in the Phacility cluster.

The next diff tailors the help on the "Auth Providers" and "Create New User" pages to say "PHACILITY PHACILITY PHACILITY PHACILITY".

Test Plan: Browed to "Auth Providers" and "Create New User" on instanced and non-instanced installs, saw appropriate guidance.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9304

Differential Revision: https://secure.phabricator.com/D16861