Closes #14690: Pretty-format JSON fields in the config form

[markkuleinio]

Fixes: #14690

Adds encoder attribute in ConfigParam that is set to a custom JSON encoder if field is a JSONField.

Replaces the Django-original JSONField params with our own JSONFields, to get pretty JSON printing in the configuration editor.

[jeremystretch]

We actually have a custom JSONField class that we use elsewhere to effect cleaner rendering, so ideally we'd reuse that for the relevant config parameters. However, importing the class directly raises an AppRegistryNotReady exception, because the class needs to be imported before the utilities app can be registered.

One option would be to swap out the field class in ConfigFormMetaclass.__new__(), changing param.field to our custom JSON field wherever it's set to Django's stock JSONField. It's a bit of a hack, but IMO reasonably clean given the constraints.

[markkuleinio]

I needed to chew this a while 😁 Totally missed the custom JSONField earlier. Let's try again

[jeremystretch]

Thanks @markkuleinio!