Previously, if one had started working on a message from a draft, and
then schedules it to be sent later, the draft would not be cleared
when the server accepts the request to schedule the message.
This case handled client receiving an `add` event before
scheduled_messages was initialized. Since `server_events`
is initialized after scheduled_messages is initialized,
this case was not possible.
Fixes#25340
This means that we now schedule the message simply after selecting
time if the message is valid.
Also, editing scheduled messages will now delete the scheduled
message and open compose with scheduled message.
Tim Abbott
Aman Agrawal
Lauryn Menard