Script: fix a bug with the hamburger menu.

* When the hamburger was deactivated and the window size was resized,
the menu was still hidden. Now, we observe the window size and reset
the margin-left of the menu when the size of the window becomes larger
than 500px (the small-screen threshold).

* We also change the toggle to prevent a double-click the next time the
hamburger menu is activated AFTER a resize.