Refactor userInfo to provide accounts data

[theely]

The FirecREST UI utilizes the /userInfo endpoint to render the account/project drop-down.
However, the /userinfo data only contains the group information, that at CSCS happens to be aligned with the accounts.

    "user": {
        "id": "1001",
        "name": "fireuser"
    },
    "group": {
        "id": "100",
        "name": "users"
    },
    "groups": [
        {
            "id": "100",
            "name": "users"
        }
    ]
}

Instead, we should add a dedicated field for accounts only:

{
    "user": {
        "id": "1001",
        "name": "fireuser"
    },
    "accounts": [
        {
            "name": "staff"
             "default": true
        }
    ]
    "groups": [
        {
            "id": "100",
            "name": "users"
             "default": true
        }
    ]
}