Fix sending of duplicate emails

The save method on the LTARequest model was overridden and used to send e-mails, but the change detection (which queried the database) was not good enough:

• now, update_fields in the LTARequest.save method is inspected to make sure that the status code is being changed (previously, it was also reacting to the request_token field being updated)
• in the LTARequestStatus.save method (which calls LTARequest.save), change detection is also added to prevent duplicate save calls
• a bit of useful logging has been left in place

Unit tests were added to check if a celery task is being sent (or not) and whether an LTARequestStatus correctly saves an LTARequest.