Abstract JSON saving React form components to have a common back-end

[NightJar]

From point 6 in #57

@robbieaverill suggests

React component architecture: similarly to the PHP FormFields, these React components all read from initial JSON value prop or state values, and store the state value in a hidden input as serialised JSON. We could centralise some of this logic too, potentially with an HOC, helper functions in a separate file or even a parent component.

@ScopeyNZ thinks this is a good idea

Yeah that seems like a pretty quick win to me. Updating the value might be a little more ambiguous though I guess. It'll be a bit like redux-form where the HOC has to supply an onChange prop that can be used to mutate the value of the field somehow.

Summary
This module consists of a number of form fields that read, parse, and present JSON blobs in a view - which then has a number of operable elements that can configure that blob and save it back to the system as a form submission.

There are common elements in this in that there is

• the initial load of JSON data
• saving JSON data
• interaction elements to configure that JSON blob

At least two of these three points could be abstracted to use the same logic (loading & saving), and perhaps helpers could be made for the third point if it is not already a part of the saving logic.

[robbieaverill]

Do we still want to do this? I can happily close it TBH

[ScopeyNZ]

As with the other issue. I'd like to see it on admin. Ambivalent about it being done here.

[NightJar]

I think it's not necessary now, but I don't think it's worth closing either (unless as @ScopeyNZ says, it's moved to silverstripe/admin instead). It's certainly something that should be tracked.