Simple Domain Access Integration

Thank you for this great module! I needed a simple integration with the Domain Access module, so I created the attached module. This does simplify the Registration Code UI for my specific use case (remove the ability to create bulk codes from the form, and default to unlimited use of the code), however I post this here in case anyone else has a similar requirement.

Use this module when you only want to assign users to domains based on a single registration code. You can select which domain a registration code can be used on and which domains a user is added as a member of when the code is used.