Allow AuthenticationProviderConfig to be enabled and disabled

Summary: Ref T1536. Nothing too exciting here, one TODO about tailoring error messages.

Test Plan:
{F46403}

{F46404}

{F46405}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6204