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 
@bummytime - Not really a correction but just a thought: I'm seeing this setting is configured to YES when a new user is created.  I was wondering if we want this setting affected in any of these circumstances:

- User logs out and another user logs in.
- New user logs into another device.

IMHO we can go for the easy route and not worry too much, but some users may complain they have a different editor on different devices so I wanted to bring this up for a brief discussion.