Make navbar collapse on smaller screens

This has some issues.

1. Clicking log out does not close the menu. This is, presumably because the event linking isn't performed since the Log Out link is not there when the header is first created.

2. In Book Library, the menu is hidden behind the filter bar. Setting z-index: -1 on .filterBar makes this work but then causes the filter bar links not to work. I'm assuming when the navbar is collapsed, we want the sidebar collapsed, too, though.