[reports-service] Endpoint for Redux Devtools import

Summary:
Keyserver has an endpoint for importing Redux Devtools state. This diff adds a similar endpoint to reports-service.

The function for modifying JSON is based on: this keyserver function.

Depends on D8971

Test Plan:

1. Uploaded an error report to the service
2. Opened the endpoint in the browser - a report-{uuid}.json file was downloaded
3. I was able to open the file in Redux Devtools and browse the state

For non-error reports, the service returned HTTP 415.

Reviewers: michal, patryk, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8972