Change script id delimiter so it's not an underscore

This allows us to have underscores in view columns, and to easily write formulas that can introspect. 

```python
def libConfig_getFieldId(script_id):
    """ Parse field id from script id

    Turn this: `field-_-frmConfiguration-_-construction_date-_-formula`
    into this: `construction_date`
    """
    script_type, form_id, field_id, tag = script_id.split(SCRIPTID_DELIMITER)
    return field_id
```

Without this, you have to tiptoe around the fact that field and form names often also contain underscores.