Standard smallest screen is 600px (#520)

For devices that have a screen size so small they have to use the `max-width` property in CSS, the standard of a screen-size that has to be used here is `phones 600px and smaller`, not `900px`, which would have to use the `min-width` property instead, that would mean a widescreen size, because `min-width` means `a screen size of 900px, or more`, including desktops.

static/css/main.scss

@@ -18,7 +18,7 @@ html {
 @import 'forms';
 
 /// Small screens
-@media screen and (max-width: 900px) {
+@media screen and (max-width: 600px) {
   @keyframes menuOpening {
     from {
       transform: scaleX(0);