Do not rely on specific JSON structure

## Expected behaviour

The admin tool can be used with any Hydra-conform API, that have the correct semantics.

## Actual behaviour

The tool relies on specific JSON structures on some places, e.g collections need to have a property "hydra:member" on top level of the JSON structure. APIs with other namings or nesting in `@graph` fail. Same goes for "totalItems" and I suspect several other places.