Allow to add neuroglancer dataset in wk frontend

[jstriebel]

If webknossos has a py-datastore attached, one should be able to also add a neuroglancer dataset to one of the py-datastores in the "Add / Upload Dataset" View (maybe in an extra tab)
(https://demo.webknossos.org/datasets/upload).

py-datastores can be identified at /api/buildinfo.

At some point (probably directly from frontend, possibly also via wk), this should POST to the datastore's /data/datasets endpoint, with some json like:

{
    "neuroglancer": {
        "<orga>": {
            "<dataset_name>": {
                "layers": {
                    "<layer_name>": {
                        "source": "<link, e.g. gs://neuroglancer-fafb-data/fafb_v14/fafb_v14_clahe>",
                        "type": "<image or segmentation>"
                    }
                }
            }
        }
    }
}

type can be image or segmentation. source should probably start with gs://, http:// or https://, but the datastore will fail anyways on wrong inputs. The organization should be the one of the current user.

So the user needs to choose
a) to use the neuroglancer backend
b) the dataset name
c) a number of layers, their names, sources and types

[jstriebel]

see scalableminds/webknossos-connect#44