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 could never get the styling right when I was working on this at first, and I'm still not able to get the styling to a place that I'm 100% happy with. (could be missing something easy though, not that familiar with this)

I also tried to work on a way to do a manual merge as well, somewhat like what git gives you, but that didn't work out either. (although now this sounds dangerous to me if the user doesn't know what they are doing)

I could see showing a raw XForm diff being useful for advanced users with small changes. I'm not sure how much once differences in server and local become large.