fix centering of installation assistent, fixes #2550

Closes #2550

Merge request studip/studip!1723

resources/assets/stylesheets/scss/installer.scss

 @import "../mixins/colors.scss";
 
+body#install {
+    grid-template-columns: auto;
+    justify-content: center;
+}
+
 .stage {
     $image-path: '../images/';
     $icon-path: '#{$image-path}icons/';