webgateway: use list comprehension instead of map (fix for Python 3)

[JonnyJD]

to fix JSON serialization,
as map() is an iterator in Python 3 now:
https://docs.python.org/3/library/functions.html#map

The fixed error is:

{"message": "Object of type 'map' is not JSON serializable", "stacktrace": "Traceback (most recent call last):\n  File \"/opt/omero/web/venv3/lib/python3.6/site-packages/omeroweb/webgateway/views.py\", line 1448, in wrap\n    return JsonResponse(rv, safe=safe)\n  File \"/opt/omero/web/venv3/lib/python3.6/site-packages/django/http/response.py\", line 530, in __init__\n    data = json.dumps(data, cls=encoder, **json_dumps_params)\n  File \"/usr/lib/python3.6/json/__init__.py\", line 238, in dumps\n    **kw).encode(obj)\n  File \"/usr/lib/python3.6/json/encoder.py\", line 199, in encode\n    chunks = self.iterencode(o, _one_shot=True)\n  File \"/usr/lib/python3.6/json/encoder.py\", line 257, in iterencode\n    return _iterencode(o, 0)\n  File \"/opt/omero/web/venv3/lib/python3.6/site-packages/django/core/serializers/json.py\", line 124, in default\n    return super(DjangoJSONEncoder, self).default(o)\n  File \"/usr/lib/python3.6/json/encoder.py\", line 180, in default\n    o.__class__.__name__)\nTypeError: Objec* Connection #0 to host 192.168.56.125 left intact
t of type 'map' is not JSON serializable\n"}

for example on a request like this:
/webgateway/dataset/1024/children/

It would also be possible to just wrap a list() around, but list comprehension is less complex and also used in other parts of the same file.

[will-moore]

Fix works and code looks good. Thanks @JonnyJD.