Goal: Reduce need for several round trips to REST server

Abstract:
1. Let caller of REST api specify that links in a document should be included in the document itself to avoid server round trips.
Do this by inlining includes on the server for best DEV setup performance, and with cache tags for production cache invalidation covered
2. Take advantage of this in UI

Do this in similar way as specified in JSON API for familiarity, however with media type structure we have today.

m2

See REST improvment epic for stories needed to better take advantage of thus form UI and headless use cases.

m3

Possible future m3 could be to look into getting more cache efficiently by instead of inlining the includes, use one of:

• ESI (cache efficiency, but depends on feature to set media type or url, like .json, as it won't work with xml because of xml structure)
• HTTP/2 Server Push (cache efficient, however not supported by Varnish yet, and client would need to support it)