JSONFormField (through JSONField) had a pretty underwhelming editing

experience when used in forms. The text field would just contain a big

blob of serialized JSON content, which was pretty hard to read and

modify.
Now, the text field shows a pretty-printed representation of the JSON

data, with all keys sorted to make it easier to find the key you want.

When saved, the resulting data is once again condensed, but with the

keys remaining sorted in the database.
In order to do this, the old encoder argument for these classes has

been deprecated, with new encoder_cls and encoder_kwargs replacing

them. This allows the classes to decide when the encoder is constructed

and to provide additional arguments to control the behavior. Any

consumers providing a custom encoder today will not see the new editing

behavior, falling back on the legacy behavior instead.

Manually tested loading a JSON structure and editing it in the Admin UI.

Saw that the data was nicely indented and sorted. Saved it and saw that

the data was condensed in the database, but still sorted.
Unit tests pass.