This will help us remove scheduled message and reminder logic
from `/messages` code path.
Removes `deliver_at`/`defer_until` and `tz_guess` parameters. And
adds the `scheduled_delivery_timestamp` instead. Also updates the
scheduled message dicts to return `scheduled_delivery_timestamp`.
Also, revises some text in `/delete-scheduled-message` endpoint
and in the `ScheduledMessage` schema in the API documentation.