Implemented EZP-23640: Update demo content for eZ Publish 5.4 release

Jira issue: https://jira.ez.no/browse/EZP-23640

Reference website: http://ezp54.demodata.ez.no

Main changes:
1. Added Section Premium content (identifier: premium_content, id: 7)
2. Added "Premium content" to allowed content/read sections for Anonymous Role
3. Created new Role "Subscriber", without policies
4. Created new User group "Subscribers"
5.  Assigned Roles Subscriber and Anonymous to Subscribers group
6. Assigned Section Premium content to /Getting-Started/Selected-Features/Getting-Started-with-eZ-Publish-Platform
I'm a bit skeptical about the meta attribute group, to be frank. But it might be subjective.