Add feature to restrict E-Maildomains

Enable the restriction of access via GoogleLogin to allowed E-Maildomains.
Introducing $wgGLAllowedDomains as config array in LocalSettings.php
to specify those allowed E-Maildomains.

With $wgGLAllowedDomainsStrict in LocalSettings.php can configured, if
the domain must exactly match (test.example.com don't macth example.com if
activated).

Added configuration variables into GoogleLogin.php with short explanation
and a default value.

Bug: 67220
Change-Id: I00b9cb9b844d5fbebe3bcb12f101d91ff609014c
Merge "Add feature to restrict E-Maildomains"