Switch CSS -> LESS, refine markup for less styling, etc.

• Redo the header and sidebar in LESS. So far the Refreshed v4 changes haven't affected the content and footer yet. Those are largely usable but not as fully functional/good looking as before (which is fine since we need to redo them anyway). For those reasons, the footer and fixed toolbox are hidden for now.
• Change hover effect for site navigation dropdown and collapsible
• Adjust the "containing..." option on the search dropdown: now it always sticks to the bottom of the dropdown instead of being hidden behind a scrollbar
• Adjust markup for easier styling
• Remove duplicate IDs: some matching items in the main, explore button, and sidebar copies of the header category dropdowns all had the same ID
• Get rid of divs inside labels because those aren't allowed per the spec: https://www.w3.org/TR/html5/forms.html#the-label-element
• Change section, aside, header elements to div as those elements carry semantic meaning about the page's content and probably shouldn't be used for the interface

Change-Id: Iea8e0bf4774e760f958a793cb46273acebfd171e