Make toggle float on top of the other content

Except for the add-on verification which would otherwise cover up the
"Install" button.

wiki/src/install/inc/stylesheets/assistant.css

@@ -220,6 +220,12 @@ ul {
   margin-right: auto;
   margin-top: -15px;
   width: 80%;
+  position: absolute;
+  z-index: 1;
+}
+
+#download-and-verify div.toggleable {
+  position: relative;
 }
 
 #pagebody #content a.toggle  {