INFRA: add dropdown

[lwasser]

TODO:

• Fix about peer review link in drop down - page doesn't exist now
• Drop down links currently aren't working - not sure why but asked on SO
• Added aria elements for accessibility
• Add a media query to hide the drop down at the same width where the hamburger disappears.

---

What's left

• essentially move the JS for the nav to a .js file and load it in the appropriate place (need to research where that should be)
  it sounds like i should add it to the footer right above the </body> tag .. can do that next.

[lwasser]

new 404 page for bad links... i think this might work throughout the site. might be nice to add some more link structure to this page.