Add Loginreplacement and disable create account

New configuration variable $wgGLAllowAccountCreation to disable (or force
to enable) that the user is allowed to create a new wiki user account. Default
to user right "createaccount".

Feature added to replace the entire MediaWiki own login frontend with
GoogleLogin (MediaWiki accounts still needed). Configured with new
configuration variable $wgGLReplaceMWLogin. Default to false (both, MediaWiki
Login and GoogleLogin is enabled). If set to true, registration of new accounts
is only possible with GoogleLogin (for logged in users (and admins), too).

Change-Id: Ic082e4625798cfe6487ca99850e17acdab9ba907
Merge "Add Loginreplacement and disable create account"