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
@bdunogier It seems Roland moved the attribute to the button on purpose as it is `meta` category, not blocker, but nice to know.