EZP-22324: BDD tests for premium content

> [EZP-23324](http://jira.ez.no/browse/EZP-23324)
> Makes no sense until #116 is merged, and matching democontent is added

Adds feature scenarii for premium subscriber content.

## TODO
- [ ] Update demo content
- [ ] Merge #116
- [ ] Adapt to the actual premium content that will be added to the DemoBundle