Use $() instead of mw.hook( 'wikipage.content' ).add()

mw.hook( 'wikipage.content' ) is fired on every live preview.

Bug: 69674
Change-Id: Ia7a6b35721b9ce62b4c8d523817863b51065ea51
Merge "Use $() instead of mw.hook( 'wikipage.content' ).add()"