campaigns: implement licensing

Created by: LawnGnome

This is the implementation of #15351 (closed).

@rrhyne has provided a Figma design, and notes these changes:

• Home page gets a simple CTA message
• Preview page also gets a banner
• Preview page apply spec button is disabled, with a tooltip describing why.

In terms of the implementation, we should check this in the GraphQL backend using the new licensing package and expose a new field akin to the user token check @eseliger is adding in #15636. (Or, put another way, I'm going to steal as much of that as I possibly can.)