there are three steps to this
1. Create a job
* Starts in status 'ready to send'
* Created by None, since it's from the API
* original file name 'letter submitted via api'
2. Create a single notification for that job
* job_row_number 0
* client reference if provided
* address line 1 as recipient
3. Trigger the build_dvla_file task
we know that all the notifications have been created for this job
(since we just created them ourselves synchronously), so this will
just create the dvla-format file for the job, and upload it to s3.
