Make Special:HideBanners set a P3P header for IE

This patch adds the new global variable
$wgCentralNoticeHideBannersP3P for setting a site's P3P header.

If that variable is set to false, we will:
- Send an invalid header.
- Add a Special:HideBanners/P3P page with an explanation of
the invalid header.

This follows a similar practice in the CentralAuth extension.

Change-Id: I34c8937165dcbc425d546e7538300a988b99a4f8