* Avoid double JSON encode/decode for socket.io
socket.io (python and js) already has a built in mechanism for JSON encoding
and decoding messages over the websocket. To use it, we pass a custom `json`
namespace which uses `format.json_dumps` (leveraging reflex serializers) to encode the
messages. This avoids sending a JSON-encoded string of JSON over the wire, and
reduces the number of serialization/deserialization passes over the message
data.
The side benefit is that debugging websocket messages in browser tools displays
the parsed JSON hierarchy and is much easier to work with.
* JSON5.parse in on_upload_progress handler responses
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