refs https://github.com/TryGhost/Team/issues/2339
- Includes a new pattern in the job manager that allows us to properly
await jobs.
- Added new convenience mocking methods to stub settings
- Tests the main flows for bulk sending:
- Sending in multiple batches
- Sending to multiple segments
- Handling a failed batch and retrying that batch
- Fixes bug in batch generation (ordering not working)
In a different PR I'll add more detailed tests.
closes https://github.com/TryGhost/Team/issues/2309
- adds new mailgun provider to send out batch emails
- updates sending service to send email id for mailgun provider, allows tagging mail with email id
