fix #4877: add customizable keybind for closing modals

[ian-h-chamberlain]

Fixes #4877

I got frustrated enough by this issue to try tackling it myself! This creates a new keyboard shortcut for closing Modals, which defaults to Escape (the previously hardcoded behavior). I'll probably set it to Ctrl-C for myself to restore the old-old behavior before #4877 was reopened.

There are a couple other places that had 'Escape' hardcoded for some kind of keydown event, but it seems mostly harmless - AFAIK this was the biggest painful hardcoded shortcut.

A couple notes on the implementation — I'm happy to add a test somewhere if that's wanted, but I wasn't sure exactly where to look. I did manual testing and it seems to work but I could use a pointer if there's a UI test that could correspond to this change.

Also, happy to take feedback on the description of the keyboard shortcut as well as the help text, I just plumbed in something quick to verify functionality.

[kobenguyent]

FYI @ian-h-chamberlain #6518 (comment)

[ian-h-chamberlain]

Dang, thanks for the pointer, I guess this change might not be accepted either. I'd ask the maintainers to consider it as a stopgap until the refactor they mentioned is ready, but if that's happening soon then I suppose it's fine, as long as the use case in #4877 is still supported!

Guess I should have tried to fix this much longer ago and maybe it could have worked in the meantime 😅

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