Add hook to allow template tweaks before rendering

Things like Zero want to override certain data keys. This allows
them to

e.g.
	public static function onMinervaPreRender( $template ) {
		$template->set( 'privacy', 'New privacy label' );
		return true;
	}

Change-Id: I89a89e9994b074d066e2a318aad0a160a634d7ae
Merge "Add hook to allow template tweaks before rendering"