Only new mobile app users should see the new editor

Fixes https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/126

If DEBUG :point_right: New Editor
If INTERNAL_BUILD :point_right: New Editor
If App Store Build + new user account created in iOS app :point_right: New Editor
If App Store Build + existing user :point_right: Old Editor
If App Store Build + any other condition :point_right: Old Editor

/cc @diegoreymendez 
I'd go ahead an merge this code.  This is something we can talk with @thianhlu and eventually add a new ticket if necessary.

:squirrel: