Change: Differentiate between user-id and username

With oCIS user-id and username are not the same as is the case in ownCloud 10.
We've started differentiating between them to correctly display all information in the accounts page.
If the username is not available (oC10), we fall back to using user-id as the username.

https://github.com/owncloud/ocis/issues/440
https://github.com/owncloud/web/pull/3938