Merge branch 'dropdown_icon_not_clickable' into 'master'

Dropdown icon not clickable

Fixes #1642 by using `pointer-events` property supported by [modern browsers](http://caniuse.com/#search=pointer-events)

See merge request !1319

app/assets/stylesheets/generic/markdown_area.scss

@@ -20,6 +20,7 @@
       opacity: 0;
       font-size: 50px;
       transition: opacity 200ms ease-in-out;
+      pointer-events: none;
     }
 
     .div-dropzone-spinner {