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