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.
Updates the objects in the API for scheduled messages so that those
for stream messages return the `to` property as an integer since it
is always the unique stream ID and so that those for direct messages
do not have a `topic` property since direct messages never have a
topic.
Also makes small update so that web app scheduled messages overlay
has the correct stream ID.
