Use external contexts when serving JSON-LD results

This changes the format served to compact JSON-LD with external contexts.

See #72, #182, #183, and #207 for details on the contexts.

The context documents are moved and altered in some minor aspects:

- Moved into lodmill/lodmill-ui to be loaded and served directly from the API
- Renamed to use our index IDs for easy automatic loading of required context
- Use `.json` suffixes for ease of usage with JSON tools (e.g. editors)

Deployed for testing at http://staging.api.lobid.org

Note that the external context URLs are hard-coded to `api.lobid.org`, but since this is not deployed to production yet, and the index names on staging have `-staging` suffixes, they will not resolve. To test serving of the external contexts, add `staging.`, e.g. http://staging.api.lobid.org/context/lobid-resources-staging.json