Use RelaxedContainerJSONEncoder & FlaskRelaxedContainerJSONProvider for json encoding where appropriate

[risicle]

This bumps notifications-utils to 124.2.0 and uses RelaxedContainerJSONEncoder to encode json payloads we're sending via requests. This should address any problems arising from InsensitiveDict no longer directly inheriting from dict.

It also sets FlaskRelaxedContainerJSONProvider our flask app's json_provider_class so we get the same robustness when returning json via e.g. jsonify.

There are also a couple of other random places we used json.dumps which I've replaced.