Permission control

@dberenbaum feedback

> How can I have permissions/control over who can register/promote to different environments?

Need to think about the way. We need to reuse GH permissions, as I guess we can't setup another layer of security around this. The question is, what scenarios does it solve and how implementations will look like.

Another question is how it will be blended with Studio permissions/RBAC in future, if we'll decide to make some. As Model Registry is a central part of some workflows, we'll need to come up with about something here.