initial-setup: Add battery icon

There is no reason why there shouldn't be a battery icon in initial-setup,
knowing how much battery you have left is useful in during setup as well.

This also fixes an exception in the lock screen caused by the combined icon
not finding the battery one.

https://bugzilla.gnome.org/show_bug.cgi?id=696212

js/ui/sessionMode.js

@@ -84,7 +84,7 @@ const _modes = {
         panel: {
             left: [],
             center: ['dateMenu'],
-            right: ['a11yGreeter', 'keyboard', 'volume']
+            right: ['a11yGreeter', 'keyboard', 'volume', 'battery']
         }
     },