Implement ISetDisplayNameBackend in the user backend

[julien-nc]

closes #965

This fixes incorrect photo in system addressbook vcard (it was always an avatar based on the previous display name or the user ID for new users).
The server apparently behaves differently if the user backend is a ISetDisplayNameBackend one or not.
This change also simplifies the provisioning service.

To reproduce the initial issue:

• create a user on the Idp side with a user ID equal to aaa for example, a first name of Jane and a last name of Doe
• Login with this user via user_oidc
• Check the system address book in the contacts app. The Jane Doe contact's photo is just an A. It should be JD.
• Change the first name to Zen on the IdP side
• Login again
• The contact photo is now JD and should be ZD