Fix responsiveness below 940px (not tested)

Below a width of 940px, the direct `section`-children of `#page` didn't get a width smaller than 940px. This caused the elements inside to exceed the screen horizontally. Adding a `width: 100%;` to `#page > section` fixes this. `width: auto;` or something similar would do the trick, too. This is not tested thoroughly, only experimented with in the developer console of Chrome. I can't imagine why it won't work elsewhere, though.