Fix checkbox toggle functionality

[jamieburchell]

Fixes #282

Also standardises the use of var and $ instead of const and jQuery respectively and generally simplifies the logic.

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[priethor]

Thanks for the contribution, @jamieburchell !

I can reproduce the issue in trunk and can confirm that this patch elegantly solves it.

I've added a couple of unit tests to prevent future regressions; they fail in trunk but pass in this PR, as expected.