Fix Dashboard modal close button position.

When the page was scrolled down, the modal close button was
invisible. This patch sets the close button to `fixed` positioning
so it's always in the same place on the screen regardless of scroll,
just like the modal background.

src/scss/_dashboard.scss

@@ -74,7 +74,7 @@
 .UppyDashboard-close {
   @include reset-button;
   display: none;
-  position: absolute;
+  position: fixed;
   top: 12px;
   right: 12px;
   cursor: pointer;