Make list of past semesters a hamburger on small screens

[NicMcPhee]

When you view the senior sem website on a phone, the "menu" of past semesters takes up nearly two full screens before you get to the "real" content (e.g., info about the current semester). It would be good to have that in some sort of menu/hamburger kind of thing instead of always expanded and taking up some much space.

[floogulinc]

There are some other mobile issues, namely that the tables of papers don't really fit on mobile screens.