Write migration transitioning JSON data from service usage events to new format

[cortadocodes]

Feature request

The current format uses "type" as the key for the event kind while the new format uses "kind". We currently have backwards-compatible methods and properties on the Question model and QuestionAdmin, but it would be better to migrate the data. In some deployments, this migration will involve gigabytes of data (there are lots of events) so it would need to be batched instead of loading everything into memory at once.