IBX-7222: Added 4px line to dropzone placeholder

Author: Gengar-i

src/bundle/Resources/public/scss/mixins/_drag-and-drop.scss

@@ -537,7 +537,7 @@
 
 @mixin drag-item-placeholder {
     display: flex;
-    border: calculateRem(1px) solid $ibexa-color-info;
+    border: calculateRem(2px) solid $ibexa-color-info;
     border-radius: calculateRem(8px);
     margin-top: calculateRem(8px);