consolidate JSON saving form fields with a base class

[NightJar]

As per point 1 in #57

@robbieaverill points out

PHP FormField implementations: all three FormFields are essentially composite React component fields which read and write their value via serialised JSON. We could probably centralise some of the logic for this between the three classes.

@ScopeyNZ follows up

Good idea. I almost feel like there could be something in core for this JsonComponentField or something. You could just extend it and define what component is used...

@NightJar continues

A base "JSONField" or "JSONStoreField" or something - one can the use setComponent or define services for using different react components.

In summary
Create a form field type that will save JSON blobs to the database. Then continue to replace ResourceLocatorField, ResultConditionsField, and PresentedOptionsField (in an upcoming PR) with this base field, setting the react component property to use the relevant view.

[robbieaverill]

I don't think it makes sense to do this until #25 is completed, since ResultConditionsField just stores its JSON value as a string which is entirely handled on the frontend - there's no logic there. We'll otherwise be refactoring for one use case (ResourceLocatorField) at this stage.

[robbieaverill]

Similarly to the other issue, do we still want to do this? I'm happy to close it

[ScopeyNZ]

I think it's not that important on this repo. It's a good idea for silverstripe-admin though.

[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.