Always remove snapjs-* css classes when menus close

I made it so the `snapjs-left` and `snapjs-right` body classes only stay on while the menu is open.

Use case: I want to make the content of my menu be `display: none;` while the menu is hidden, for performance.

Now I can do this:

```css
.left-drawer {
  display: none;
}
.snapjs-left .left-drawer {
  display: block;
}
```