Navbar doesn't play well with sorta narrow windows

[Joshua Shapiro]

The navbar we have doesn't play super well on narrow displays; it goes to two lines at some points, overlapping and cutting off content and the TOC:

Originally posted by @jashapiro in #323 (comment)

For reference, the normal navbar currently looks like this:

In a very narrow window we get a "mobile" layout:

But we fail in between...

This will probably require a look from the dev team to help us sort out, and as such, it is likely to be low priority for now.