requires https://github.com/TryGhost/Ghost/pull/12192
- added initial `EmailBatch` and `EmailRecipient` model definitions with defaults and relationships
- added missing `post` relationship function to email model
- fetch member list without bookshelf
- bookshelf can add around 3x overhead when fetching the members list for an email
- we don't need full members at this point, only having the data is fine
- if we need full models later on we can push the model hydration into background jobs where recipient batches are fetched ready for an email to be sent
- bookshelf model instantiation of many models blocks the event loop, using knex directly keeps concurrent requests fast
- store recipient list before sending email
- chunk already-fetched members list into batches and insert records into the `email_recipients` table via knex
- chunked into batches of 1000 to match the number of emails that Mailgun accepts in a single API request but this may not be the absolute fastest batch size for recipient insertion:
| Batch size | Batch time | Total time |
| ---------- | ---------- | ---------- |
| 500 | 20ms | 4142ms |
| 1000 | 50ms | 4651ms |
| 5000 | 170ms | 3540ms |
| 10000 | 370ms | 3684ms |
- create an email_batch record before inserting recipient rows so we can effeciently fetch recipients by batch and store the overall batch status
