Move sidebar menu into top level navigation

I need someone with more bootstrap/CSS fu to take a look at this: I’m considering including the side bar navigation into the top level menu, which would then collapse to a hamburger menu on small view ports. Randomly adding and removing paddings worked to a limited degree – I’m sure there’s a better way. Also, comments as to why this might be totally stupid are welcome. /cc @aheusingfeld @FND @martinei @jannikolai