[change:x509] Replace thirdparty JSONField with Django built in JSONField

[nemesifier]

We have to work on these first: https://github.com/openwisp/openwisp-controller/issues/1061 https://github.com/openwisp/openwisp-monitoring/issues/673.

The JSONField we currently use is just a text field with JSON validation and formatting, which has served us pretty well but hasn't received updates in 5 years: https://github.com/rpkilby/jsonfield/.

The Django JSONField has been around for some time and seems supported on all the databases we use, hence can switch to this field.

We'll need to pay particular attention to ensuring data is automatically migrated without loss.