layout/menu: don't show uncollapsed loading menu

We have a no-JS-friendly CSS showing the menu uncollapsed unless JS
is fully loaded. Even if the page loads pretty fast this looks still
ugly in JS-enabled browsers.

This patch adds a JS-snippet removing the no-js class from the HTML
element before the rendering engine reaches the body. You'll see no
more flickering in your JS-enabled browser.

fixes #6307