Re-registering onBeforePageDisplay hook

Change I032208726699d104c8ee76ef97e3f61b5f78f334 seems to have accidentally removed the
onBeforePageDisplay hook from the Wikibase.php file preventing the native search box
being replaced by the entity selector widget.

Change-Id: Ib515bdb8ddc01072f5965b30aa12556a6821c9e9
Merge "Re-registering onBeforePageDisplay hook"