update z-index for bottom-nav

[zkamvar]

This will make sure that the dropdown menus appear on top of the sidebar
on xxl screens.

It turns out that it was a z-index issue, but not of the .dropdown-menu
class. It was the z-index of the .bottom-nav class (and why it's called bottom
nav is because in the header the top nav is for language and instructor view)
that I needed to set to z-index: 3.

The reason why this happened is because the z-index depends on the position
of the element. In this case, the position was relative to the parent element
, which was the nav.bottom-nav, and this was set to have a z-index of 2 in
the document, but the sidebar in div.container > div.sidebar-col > div.sidebar also has a z-index of 2, which means that since it comes after the
nav.bottom-nav, it still appears closer to the user.

https://css-tricks.com/almanac/properties/z/z-index/

This will address #35