Quite some changes

* DB changes (removed some plural + added few tables)
* Added user authentication + management + access right check in all the pages
* Added device group management
* Added device association to a device group (but it is now visible yet)
* Added some updated screenshots
* Removed /device/register and /device/properties as they don't make much sense anymore
* Added some sample client code so that we can easily embed it existing devices, factory installation units, etc.