Synchronize drafts

[ghost]

It would be cool to add this feature

[tosh]

To add more context (copying over from #1958)

Bug description

It would be great if I can start typing a message on iOS and then continue typing on the Desktop.
Telegram behaves this way and it helps when switching between devices.

Steps to reproduce

1. Start to type a message on Signal for iOS
2. Don't send the message yet
3. Switch to Signal for Desktop

Actual result:
Text input field on Signal for Desktop is empty

Expected result:
Text input field on Signal for Desktop contains what I typed on Signal for iOS
so I can continue typing my message.

Platform info

Signal version:
1.1.0

Operating System:
macOS

Linked device version:
iOS 2.19.3.3