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.

## Summary
Content from the `Premium` section will only be fully visible by members of the `Premium subscribers` group. Users who don't match this requirement will be shown a teasing page, showing a specific attribute's content, as well as a piece of text explaining premium membership.

## Implementation
A [custom location view provider][ViewProvider] displays a [teasing template][template] if the content is Premium (section id 7 (Premium)), and if the user doesn't have the subscriber role (id: 6).

Covered by [behat scenarii][BDD].

## 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)

[BDD]: 116/files#diff-0
[ViewProvider]: 116/files#diff-2
[template]: 116/files#diff-4
@bdunogier IMHO I believe that before the When you should have a Given. For instance

Given I am not logged in

or 

Give I am logged in as "anonymous"