hotkeys: refactor to use .on(), namespaces.

* Use .on() instead of .bind() because bind is deprecated in jquery 3.0.
* Ensure enable-js-navigation is always respected.
* Namespace keybindings so they may be disabled by userscripts with
  e.g. $(document).off("keydown.danbooru") or $(document).off("next_page").

app/assets/javascripts/utility.js

@@ -45,6 +45,12 @@
     }
   }
 
+  Danbooru.keydown = function(keys, namespace, handler) {
+    if (Danbooru.meta("enable-js-navigation") === "true") {
+      $(document).on("keydown" + ".danbooru." + namespace, null, keys, handler);
+    }
+  };
+
   Danbooru.is_subset = function(array, subarray) {
     var all = true;