Broken links in AdminCustomerUser module when using another CustomerKey than UserLogin

When OTRS is configured for other CustomerKey than UserLogin, links in AdminCustomerUser are broken because they use the 'UserLogin' attribute to get CustomerUser's data.

This patch solves the issue by adding a 'CustomerKey' attribute to the data returned by the CustomerUserDataGet functions, and using it in the links.

The patch also change the name of the corresponding column from "Username" to "User login".

Cyrille