Additional option to associate membership levels to Learndash groups

[crypticnewbie]

Currently it is only possible to assign courses directly to one ore more membership levels. However, sometimes it is more practical to be able to sell Learndash groups with a membership, e.g. when a "Premium" level will have new courses added to it as time goes by.

This way, when a new course is added to the group, it will be automatically assigned to members of the "Premium" subscription without having to add it manually to their accounts.