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
Not the end of the world, but would ideally be better if the content class attributes did not specify the category when in the default one. Less chance of breaks if someone changes settings.