Warn before overwriting work

So I attempted to do this as a PID on a flight. @orangejenny brought this up in her field report. No HQ changes necessary.

I also attempted to do some work on automatically merging conflicts: https://github.com/dimagi/Vellum/compare/concurrent-editors

While that works suprisingly ok for non conflicting changes, I had to fix enough malformed XML to not recommend merging that, but it may give a starting point.
Vellum has [XML diff output for tests](https://github.com/dimagi/Vellum/blob/overwrite-warning/tests/utils.js#L40-L45), and it works surprisingly well. Maybe we could use something like that here? For now it would only show what's about to be overwritten, but that seems better than just saying you're about to overwrite someone else's work. Maybe the diff could be hidden by default and a button on the warning dialog would show it?

Granted, showing a raw XForm diff is not very user-friendly.