Add storage for Auth configuration in preparation for moving it into a web interface

Summary:
Ref T1536. Currently, we have about 40 auth-related configuration options. This is already roughly 20% of our config, and we want to add more providers. Additionally, we want to turn some of these auth options into multi-auth options (e.g., allow multiple Phabricator OAuth installs, or, theoretically multiple LDAP servers).

I'm going to move this into a separate "Auth" tool with a minimal CLI (bin/auth) interface and a more full web interface. Roughly:

• Administrators will use the app to manage authentication providers.
• The bin/auth CLI will provide a safety hatch if you lock yourself out by disabling all usable providers somehow.
• We'll migrate existing configuration into the app and remove it.

General goals:

• Make it much easier to configure authentication by providing an interface for it.
• Make it easier to configure everything else by reducing the total number of available options.

Test Plan: Ran storage upgrade.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6196