Feature/a11y terms

[klngwll]

Follow up on #526

• Dust off subcClasses for ContentAccessibility based on schema terms. Keep properties out off KBV for now only using contentAccessibility as "container" within the Bib-frame. Easier to add more properties than remove if needed later. For now we use blank nodes.

In this PR also draft for linked terms. (Need more work)

• Add a11y-terms.ttl with terms for features, hazards, mode and mode sufficient. Not in this PR: publishing on id.kb.se
• Suggestion on form combining accessModeSufficient terms
• Links to documents forming the basis included for traceability. Definitions and labels in english have been added from these where most suitable atm.
• Add unstable status to terms indicating all things are moving, also in international groups regarding presentation and combinations of terms. This is the first step to comply with EAA. We might need to revise and improve later, and find suitable ambition for overlaps with normalization issues etc.

See FMT-26 for details.

[niklasl]

LGTM! Let's start with this and work on finalizing the terms (tacking standard alignments/cross-walks) for subsequent publication.