Allow login to be disabled for authentication providers

Summary:
Fixes T9997. This was in the database since v0, I just never hooked up the UI since it wasn't previously meaningful.

However, it now makes sense to have a provider like Asana with login disabled and use it only for integrations.

Test Plan: Disabled login on a provider, verified it was no longer available for login/registration but still linkable.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9997

Differential Revision: https://secure.phabricator.com/D14794