When users try to log out with no providers configured, warn them of the consequences

Summary: Fixes T13406. On the logout screen, test for no configured providers and warn users they may be getting into more trouble than they expect.

Test Plan:

• Logged out of a normal install and a fresh (unconfigured) install.

Maniphest Tasks: T13406

Differential Revision: https://secure.phabricator.com/D20789