rustdoc: simplify settings popover DOM, CSS, JS
* Change the class names so that they all start with `setting-`. That should make it harder to accidentally use a setting class outside the settings popover, where loading the CSS might accidentally change the styles of something unrelated.
* Get rid of an unnecessary wrapper DIV around the radio button line.
* Simplify CSS selectors by making the DOM easier and more intuitive to target.
* Remove dead settings JS for obsolete select-wrapper
e78fa8a0bc