[Comments] Prompt warning when exiting page with draft comment

[mattsoh]

Describe the problem

I'm writing a really nice, long, and detailed comment... and then I want to upload a receipt, or maybe I accidentally click another button.

Either way, my draft comment is lost... 😭

What will I do? I'm sad...

Describe the feature/solution (optional)

Do the unsaved changes warning thing or something?

[Luke-Oldenburg]

Maybe we should auto-save