Header links on mobile are easier to interact with

Moved header links to their own drop down when viewing on mobile. Changes are all via css and media queries, so no elements are added or removed. Toggling is done by adding and removing a class via javascript. This is the simplest way I could think of without changing the structure of the html. 

The dataURL I made for the menu image is a little large compared to others on the site, but I don't know how to make it any smaller. 

This is just a minor issue I ran into while using the site on mobile. Let me know if you think it's a valid concern. I'm also open to advice for styling the drop-down menu and/or changing the way it works. 

Thanks!