Fix modal css to be hidden immediately

Before the leanModal content css was applied after the js class
was added to the body element. This causes leanModals to
display for a split second before being hidden. This fix removes
the need for the explicit css rule for static pages preview.

common/static/css/vendor/normalize.css

@@ -405,10 +405,3 @@ table {
     border-spacing: 0;
 }
 
-/**
- * Hide staticpage preview on load (Studio).
- */
-
-#preview-lms-staticpages {
-    display: none;
-}