When using broser login, username was not stored so look it up

def add_username(user, apps):
    """When using broser login, username was not stored so look it up"""
    if not user:
        return None

    apps = [a for a in apps if a.instance == user.instance]

    if not apps:
        return None

    from toot.api import verify_credentials
    creds = verify_credentials(apps.pop(), user)

    return User(user.instance, creds['username'], user.access_token)