System config frontend

Hi James,

I have finished the ui frontend for the management and configuration of the user services. It is still just a dummy with no real connection to the backend. But I thought, it would be best to discuss this suggestion with you first before I invest more time to implement the necessary changes in the backend. So please take a look and let me know what you think.

![ui-draft-finished](https://f.cloud.github.com/assets/1809538/465454/5c52e0a8-b632-11e2-815a-0860b05c9502.png)

Why a frontend? It would be nice to ease the life of administrators and free them from the necessity to write property files. A frontend inside the web ui introduces a nice and self-explaning overview of the system configuration regarding authentication and permission.

Why multiple user services?
I think it would be nice to be able to configure multiple user services alongside and define an order according to which the user services will be checked to authenticate users and map groups to gitblit system roles. I could even think of one more option per user service: enable usage of remote groups to define repository permissions (would be just one more checkbox per row in the ui draft above).

Regards