Warn when leaving Edit Votes page with unsaved changes

[MichaelNMaggs]

It's possible to leave the Edit Votes page without saving changes. If a jury member does that, any changes are silently lost.

There should be a warning if the user attempts to leave the page without saving (especially as the main voting page records each vote as it's made without a separate save step).

[mahmoud]

Good catch! Almost missed this on a recent edit for a round I'm judging, thankfully I was able to go back and find the vote, change it, and actually hit save. Thanks! Looks like Pawel's seen it, so here's hoping it's fixed soon :)