Warn users before navigating away from unsaved form changes

[oxinabox]

Current Behavior

If as admim you make a change to the content page on a submission, then change the submission state, without first clicking save, then the unsaved changes are lost.

Expected Behavior

Either:

• when changing state a prompt appearing: "Unsaved changes to content, do you want to save before [accepting]?"
• or: submission state menu to be disabled if there are unsaved changes
• or: changing state to automatically save (possibly with a rename of label "save and acept"

Steps to Reproduce

1. Open a submission as admin, go to content page
2. change anything, e.g. title, or submission type
3. do not click save
4. change submission state. e.g. from submitted to accepted. Confirm with "Do it"
5. open the submission back up, observe all you changes have been undone

Context

JuliaCon often changes talk types before accepting.
For example if a talk seems like it doesn't have enough content to justify a full 30 minute talk, rather than reject it, we will often off it a 10 minute lightning talk instead.

As such changing to the submission type, then accepting is common for us.

Your Environment

• Version used:
• Environment name and version (e.g. Chrome 39, python 3.5): Firefox 88.0.1 (64-bit)
• Operating System and version (desktop or mobile): Mac, desktop
• Link to your instance, if in production: https://pretalx.com/orga/event/juliacon2021