Implemented premium subscriber content

> [EZP-22324](http://jira.ez.no/browse/EZP-22324)
> Status: Tests fail because the demo data isn't updated yet, but they pass with the proper data.

A custom location view provider displays a teasing template if the content is Premium (section id 7 (Premium)), and if the user doesn't have the subscriber role (id: 6)

## Todo
- [ ] Refactor role / premium content logic
- [ ] Update demo data with: "Premium subscriber" role, "Premium" section, "Premium subscribers" role + assignment to group, John Doe Premium Subscriber user, Zachary Zoe Member user, Premium article (update URL in test once final)