Move since and until to time_range.

def update_time_range(form_data):
    """Move since and until to time_range."""
    if 'since' in form_data or 'until' in form_data:
        form_data['time_range'] = '{} : {}'.format(
            form_data.pop('since', '') or '',
            form_data.pop('until', '') or '',
        )