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.
I looked around and I don't think we currently save the last person/people to save a file, so that would take some more HQ work to do, but it does seem like it could be useful.