Add a flag for disabling authentication via an active email authority

[mike-marcacci]

The email authority has utility as a mechanism for identification in addition to authentication. Accordingly, there are use cases that require all the current functionality of this strategy, but also require that email is not a mechanism for authentication.

This could trivially be accomplished by adding an authenticationEnabled flag on the authority that is checked in the authentication authenticateEmail mutation.

[mike-marcacci]

Note that in such a case, the user interface should avoid showing the email provider as an option.