Move LinkBegin hook handler around

In preparation to Special:MyLanguage integration to core
(I520077c931431b5919e0208f75c20b5b25f3159d), make sure that
hook handler stays in place even when the page class is overridden
by core.

Change-Id: Id28c7659e7273c29d7e57a6d0c2c4e68c8582095