REST API Content Body map has badly-formed values

The map of ContentRepresentations to ContentBody entities in the original REST API was implemented incompletely and did not allow for collapsed entities to be returned.

As a result, this map is the only known part of the public API that doesn't follow the pattern of collapsed entities being exposed via an _expandable entry in the JSON.

A change to the (ExperimentalApi) Content model object was discussed with the Confluence API team and found to be low risk to plugin developers, and will avoid future problems when more internal and external devs are using the API.