Keep dropdown menu visible when using keyboard navigation

Part of #747, prevents the dropdown menu from hiding itself
when the user tabs to an item in the list.

tildes/scss/modules/_dropdown.scss

@@ -15,6 +15,10 @@
     }
   }
 
+  &:focus-within .menu {
+    display: block;
+  }
+
   &.dropdown-bottom {
     .menu {
       top: auto;