Fix logout link code to use Userlogout instead of alias

When $wgDevelopmentWarnings = true; there are various php
error notes triggered whilst creating the login/out link
"Found alias defined for Userlogout when searching
for special page aliases for UserLogout. Case mismatch? "

Change-Id: I05e31a8b7cd8f046384ea9636466e02d897ac159
Merge "Fix logout link code to use Userlogout instead of alias"