Templatize and implement site-width tokens

[fzhao99]

Part of 2611

The goal here is for us to set the width / horizontal layout settings once at the parent level and not have to worry about that. In other words, the only spacing concern should be verticle / horizontal within the bound content block.

After chatting with Michelle, the templates (for now) should be

• a 720px (45rem) that's auto-margined for centering
  • One template for a one-column layout
  • Another template that has a two-rail 1/4 - 3/4 breakdown for pages with a side navigation (currently the final results view but potential other pages in the future)