Restrict access to certain API endpoints on a per play basis

[ingoboerner]

In the CLS Infra Project we should investigate ways to work with in-copyright material. One solution to the problem is preventing access to the full text and offering derived text formats only. To use DraCor in such a scenario we would need to have a way to prevent the API from returning data on some of endpoints when the play identified by playname is subject to copyright.

For example, in case of a copyrighted text it could be possible to download a GEXF file from the download tab, but no files, that include text; the API endpoint returning network metrics should work, the endpoint returning stage directions should not, ...