Add support for audiences defined in code

> As a developer building a feature with complex business logic for an audience definition
> I would like to be able specify audiences in code
> To prevent editorial users from changing or deleting these audience values

The interface for defining audiences is great for those audiences which users can build on their own. However, there are some common audience definitions which are cumbersome to build out, or which should be enforced in code for legal compliance reasons. An example might be "visitors from countries subject to GDPR regulation".

I think a nice feature would be the ability to register read-only audiences in code which are merged with the user-defined ones.