[REST] entities: get entries by norm data ID

[dariok]

The REST endpoint for entities currently knows “scan” – for autocomplete – and retrieval of lists of occurrences in collections or files, based on the ID of the person element.

Esp. with TEI's private URI schemes, retrieval of an entity based on a norm data ID (GND, ORCID, ...) is necessary.

A combined index for person/@xml:id, person/idno
Ideally, idno has a type that corresponds to the authority part of the private URI:

<rs type="person" ref="per:orcid:0000-0002-0118-0811">

<person>
    <idno type="orcid">0000-0002-0118-0811</idno>

[dariok]

See also #183
See also #99