Enqueue mw.util.init in document-ready even handler earlier

Enqueue mw.util.init in document-ready even handler earlier
* Previously $(document).ready(mw.util.init) was in module 'mediawiki.page.ready' (position: bottom). I've now moved this to 'mediawiki.page.startup' so that it'll be enqueued sooner.
* This making it more likely that if someone also enqueues in document-ready that mw.util.init ran before than and thus mw.util.$content populated
* Fixes bug 33711

* All this is still depends on the order in which the event handler queue is executed, which is risky. Bug 30713 will bring the solid "watertight" solution