Add optional endpoint for incremental updates

[nichtich]

Two methods (disabled by default) to modify subject indexing of a record.

• HTTP POST with full subject indexing of a record (already available in backend method updateRecord)
• HTTP PATCH to only add/remove selected subject fields (required by https://github.com/gbv/coli-rich-web/issues/5 at least for testing)

Endpoint URL could be same endpoint as HTTP GET /api/?record=${record}

Update endpoints should be restricted in the same way as JSKOS Server (see https://github.com/gbv/jskos-server#authentication) with a list of whitelisted identities.

Updates via these endpoints differ from updates via import script (#27) by import format: import scripts use internal format of backends (TSV or NTriples) , update endpoints use JSKOS Occurrences.

[nichtich]

First step to do so: #43 (probably PUT instead of POST)