Fomantic-UI's `.hidden` CSS class is badly designed.
* Checkbox elements do not need it in HTML, so this PR removes it (JS
adds the `.hidden` class back by `$('.ui.checkbox').checkbox()`)
* `menu transaction hidden` is still needed, and it should be the only
usage for the `.hidden` from now on (until they get refactored properly)
Co-authored-by: zeripath <art27@cantab.net>