informational-overlays: Focus overlay body on shortcut "?".

This focuses the body content of the informational overlay after
going to it from "?" so that you can use up and down arrows to then
scroll the content easily.

Fixes: #4480.

static/js/ui.js

@@ -124,6 +124,7 @@ $(document).ready(function () {
         callback: function (name, key) {
             $(".overlay-modal").hide();
             $("#" + key).show();
+            $("#" + key).find(".modal-body").focus();
         },
     }).get();