Collapse navigation bar into hamburger menu on small screens

[psvenk]

Continuation of #144 (comment). The tabs on the navigation bar take quite a bit of space on mobile displays, so we should collapse them into a hamburger menu depending on screen size (whether through CSS @media queries or through JavaScript).

Screenshot of the navigation header taking up 4 rows on a 640x1136 mobile screen

[andOrlando]

We should probably rethink what we need there, like if the "gpa type" button is necessary or if the grade/calculated grade even has to be on that bar in the first place.

edit: comment no longer relevant