Basic width-selecting functionality

This adds the following functionality:
- clicking an option sets the width on all preview elements
- show/hide on the width options
- `selected` class is set on clicked option

Note that most of this functionality is done via Javascript. Widths are currently set inline on the elements, but we can move those widths to CSS and classes if that's what you're into.

The major piece that's missing is the CSS for all of this. I know @micfunk was working on a new menu bar, so maybe it makes sense to wait for that before merging this in.