Handler fields in forms

**Is your feature request related to a problem? Please describe.**
Paper documents can have sections such as "applicant fills here" and "clerk fills here", it could be useful to have a similar feature in REMS forms.

**Describe the solution you'd like**
When creating fields to a form, having to select if this field is for the applicant or the handler. The handler fields could optionally be invisible to the applicant, if they contain some internal data or automation configuration, or they could be visible, so that the applicant can see some important additions from the handler.

**Describe alternatives you've considered**
It's already possible to do this by parsing remarks made on an application, but structured fields would be more efficient.

**Additional context**
- so that handlers can give structured configurations for automation
- so that handlers can fill in additional information on an application