Research: Separate forms from controllers

[chillu]

• Use existing FormFactory work in asset-admin to create a core API
• Allow form action handling outside of controllers (through FormAction closures?)j
• Identify forms without controller URLs (through injector references?)
• Create a generic FormFactory which reads/writes from a single model
• Outline how to implement more sophisticated FormFactory uses (e.g. saving to multiple models)
• Decide how to pass in context (no direct controller references)
• Decide how to deal with Form constructor (allow null for $controller argument?)
• Validate approach through a controller similar to existing LeftAndMain->schema() endpoint (no dependency on GraphQL implementation)

Outcome: RFC for required PHP API additions and changes

[tractorcow]

I've started drafting up this RFC at tractorcow#19.

[tractorcow]

Published at #6362