Move firing of "wikipage.content" mw.hook out of mediawiki.util

* Update "wikipage.content" arguments to really pass the
  content and not the entire body wrapper (no longer
  mw.util.$content).

* Move the firing of "wikipage.content" from mediawiki.util to
  mediawiiki.page.startup. It didnt really belong in
  mediawiki.util, and now that we're no longer passing it
  mw.util.$content they're no longer related in either direction.

Bug: 30713
Change-Id: I56d6491527547f898e239b531c98e903863f8999