GH-988: replace old way of managing fields with new formfield class.

[ksuhartono97]

Part 1:

• Replaced objects for formfields with FormField class for consistency
• Reworked how prefillable form fields are stored
• Adjusted UI to work with new data structures

Todo in part 2: implement ID functionality and conditional forms backend design

[ksuhartono97]

Note that this is not backward compatible. We will have to update all our old forms to contain the correct prefill fields.