Use DjangoJSONEncoder instead of custom LazyStringEncoder

[laymonage]

Cleanup of #11620. The built-in DjangoJSONEncoder can handle lazy strings and other types (e.g. Decimal, datetimes).

I've confirmed that without the cls argument, we have failing tests, and those pass when we use DjangoJSONEncoder.

[squash-labs]

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Test this branch here: https://laymonagedraftail-djangojsonen-bndlf.squash.io

[lb-]

Much better - thank you!

I thought there must be a better way when I was looking into the other PR, glad there is.