Adding a user to a subwiki should check if the user is already added

Steps to reproduce:

1. Create a new (sub)wiki and add members before the DW started
2. Go to "Administration"
3. Click on "Users"
4. Write an already (sub)wiki user and click on "Add"
5. You will see that the user listed in the "Current Members" livetable

It would be great that when you want to add an existing user in you (sub)wiki added when you created the wiki to see an message like: "The user already exists in the wiki" or something like that.