Unsaved edits lost on scroll

## Description
Unsaved work is lost if you scroll too far away from the message editor.


## Steps to reproduce
1. Open https://lyra.zetkin.org/projects/app-zetkin-org/en
2. Focus a message's editor
3. Enter some edits
4. Scroll down a page or two.
5. Scroll back up.

### Expected Behaviour
Your unsaved edits should remain

### Actual Behaviour
Your unsaved edits are lost.