During special page transclusion, save and restore context's WikiPage too

During special page transclusion, save and restore context's WikiPage too

Setting the Title by calling setTitle clears the WikiPage, and the
next time getWikiPage() is called, it will be lazy-initialized to a
different instance of WikiPage.

This is mostly okay (the behavior has been like this for years and no
one noticed any problems), but it turns out that some extensions
(ConfirmEdit) use custom properties on the WikiPage object to pass
data between different hooks, which are lost when it's re-initialized.

Bug: T207065
Change-Id: I2881895f337bcfb1f86d5fc5a994fa9b0dcc768a