Extract zhdk_integration into gem; refactor db-login

* zhdk specific controller is in a gem
* use only db-login as default
* adjust code
Fix: zhdk-user id name for login tab
Fix: zencoder test_mode? settings
Upgrade selenium webdriver
Fix version setting for xml export
Admin interface: remove admin users section

Squashed commit of the following:

commit 6ed3d44cec9ecf9f5ae06cdddd66b21bd2951968
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Thu Sep 11 18:26:05 2014 +0200

    Admin interface: remove admin users section