[chore/frontend] Refactor layout + styling for consistency

[tsmethurst]

Our frontend layout and styling evolved pretty organically, which means there's quite a bit of repetition in the styling that we use (in settings.css, and quite a but of stuff we don't use at all anymore, but which still has entries in that css file. There's also some inconsistencies in the way we wrap settings pages (some of them use one wrapping div, some of them don't).

It would be good to refactor the layout and css a bit to make it more consistent, and maybe come up with some brief guidelines for what to use where, so that future contributors know what to do.