Activate a user.

def users_activate(user):
    """Activate a user."""
    user_obj = _datastore.get_user(user)
    if user_obj is None:
        raise click.UsageError('ERROR: User not found.')
    if _datastore.activate_user(user_obj):
        click.secho('User "{0}" has been activated.'.format(user), fg='green')
    else:
        click.secho('User "{0}" was already activated.'.format(user),
                    fg='yellow')