Additional API endpoints

[nichtich]

To align with jskos-server:

• [x] /status (see #26)
• [x] /mappings/:_id where :_id is the mapping statement UUID
• [ ] /voc (see #25, requires wdtaxonomy)
• [ ] /data (alias for /concept)
• [ ] /narrower
• [ ] /ancestors
• [ ] /search
• [ ] occurrences (see #9, needs discussion)

[stefandesu]

The first two points were implemented by necessity for writing access to mappings (still on a separate branch).

[nichtich]

I created individual issues for the remaining endpoints