Use maximum of 80% for modals in large screens

When there is enough content in the modal, the height of the modal plus
its top margin could exceed the height of the viewport.

Considering a top position of 10%, a max height of 80% renders a tall
modal vertically centered, with 10% remaining space at the bottom.

app/assets/stylesheets/darkswarm/modals.css.scss

@@ -29,7 +29,7 @@ dialog
   // Medium and up - when modal IS NOT full screen
   @media only screen and (min-width: 641px) {
     top: 10%;
-    max-height: 100%;
+    max-height: 80%;
   }
 }