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
I guess config would be most clean, as it depends on how you configure it, however this is in DemoBundle, so it can be constant reflecting how data in demo content looks like.