This patch adds new array fields on `POST /tx`: `subscriber_emails[]`, `subscriber_ids[]`.
Either of these array fields can be sent with multiple subscribers.
The individual non-array fields `subscriber_id` and `subscriber_email` are deprecated.
Closes#994.
This commit adds a new API `POST /api/tx` that sends an ad-hoc message
to a subscriber based on a pre-defined transactional template. This is
a large commit that adds the following:
- New campaign / tx template types on the UI. tx templates have an
additional subject field.
- New fields `type` and `subject` to the templates table.
- Refactor template CRUD operations and models.
- Refactor template func assignment in manager.
- Add pre-compiled template caching to manager runtime.
- Pre-compile all tx templates into memory on program boot to avoid
expensive template compilation on ad-hoc tx messages.
