mediawiki.action.edit.preview: Fix for LiquidThreads hack

LiquidThreads is loading this core module when rendering an
inline edit form. Because the form doesn't have the same ID
as the one in core, this broke after becd284.

It previously worked because the click event on the button (of
which LQT did duplicate the ID from core) bubbles up to the
document body and thus trigger this private handler of the
core edit preview module.

Change-Id: I9e35d0c48b86e1c9073b68313a8a330c6ea85b86