Add 'hamburger' style drop down for mobile nav links

[EmersonFras]

Looks like this is the mobile styling. Currently just doesn't show any nav-links on mobile. I can implement the "hamburger"/drop down for the nav links in mobile here.

Originally posted by @EmersonFras in #3 (comment)

[egrace479]

Just for clarity, this does not block #1 and the initial release of the site.