Add \OC::$WEBROOT to URLGenerator::getAbsoluteURL()

URLGenerator::getAbsoluteURL() didn't take ownCloud instances running in a subdir into account. 
This caused e.g. AppFramework SecurityMiddleware to [redirect to Document root](https://github.com/owncloud/core/blob/fix_urlGenerator2/lib/private/appframework/middleware/security/securitymiddleware.php#L128) instead of to the ownCloud instance.

Tested both with ownCloud in a subdir and in the DocRoot of a VirtualHost.

I can't believe none of us noticed that in #6935 :grin: 

@PVince81 @DeepDiver1975 @karlitschek @bartv2