Set WebInstaller session variables as globals

Set WebInstaller session variables as globals

During the install process LocalSettings.php does not exist
yet so globals simply take their values from DefaultSettings.php.
The installer uses the correct settings directly for e.g. database
operations, but this blows up when something tries to use
MediaWikiServices which is hardcoded to work with globals.

Make the installer update the globals to work around this.

Bug: T187586
Change-Id: I042099c399bc58929e83178f61e6e6ca478a3ee6