Allow auth view to be served on non-default domain

[kekssw]

As different parts of the app may be served on different subdomains - e.g. browsable views and API - if would be great if flask-jwt provided an option to specify desired domain for auth view.

Additionally:

• Removed second layer of defaults for 'JWT_AUTH*' configuration parameters

  (as I don't see how those parameters may stay undefined by that point)

• Removed check for 'JWT_AUTH*' defined

  (as the extension doesn't seem to be very useful without auth view)

[mattupstate]

This is obviously useful, but you changed some functionality that I see as required. Being able to set JWT_AUTH_URL_RULE and JWT_AUTH_ENDPOINT to None allows one to disable the auth endpoint. I'm not sure why you chose to change that. However, subdomain support is good and I'd like you to add a test to verify your change.