settings: Lazy-load settings sections.

For the settings UI, we now wait until a user goes to a particular
settings section before calling the appropriate function to set
up the section (which usually involves setting up click handlers
and populating initial data).

static/js/ui_init.js

@@ -257,6 +257,7 @@ $(function () {
     activity.initialize();
     emoji.initialize();
     hotspots.initialize();
+    settings_sections.initialize();
 });