Always ensure Special:Userlogin is handled by https, if enabled

This takes advantage of the SpecialPageBeforeExecute hook to
redirect a user to the https version of Special:Userlogin if
they are initially requesting the page from http but have
wgMFForceSecureLogin set to true.

Change-Id: I23a1db6b9e79c17820556be428118121cb4640b4