Refactor formschema logic from LeftAndMain into its own abstract class

[GuySartorelli]

Currently LeftAndMain is bloated with logic for setting up formschema, but that isn't needed by most of its subclasses. This should be moved into its own class so the logic is only available where it's needed, and to make LeftAndMain easier to maintain.

Related

• SPIKE Analyse how LeftAndMain and CMSMain are put together and how they could work better silverstripe-cms#2933

Notes

• The exact name of the class isn't important - if you think of a better name, feel free.

Acceptance Criteria

• A new abstract FormSchemaController class is created, and is a subclass of AdminRouteController (see Refactor admin/* routing logic from LeftAndMain into its own abstract class #1761)
• Methods, properties, and configuration required for formschema functionality are moved from LeftAndMain to the new class (see this comment for recommendations about this)
• ModalController is a subclass of the new controller
• ModalController handles the routes which currently require creating a singleton of LeftAndMain for formschemas
• Any controller in supported modules that currently subclasses LeftAndMain but only needs the formschema logic is made to subclass the new class instead (e.g. LinkFieldController)
• Documentation is updated to indicate best practice for form schema controllers

PRs

CMS 5 PRs

• API Deprecate API that will be removed as a result of refactoring #1851
• API Deprecate API that will be removed as a result of refactoring silverstripe-subsites#613
• DOC Document deprecated API developer-docs#629

After merging reassign to Guy so they can merge up and rebase the CMS 6 PRs

CMS 6 PRs

Kitchen sink CI

• NEW Move form schema logic into FormSchemaController #1850
• API Make SubsiteXHRController a subclass of AdminController silverstripe-subsites#611
• API Make LinkFieldController a subclass of FormSchemaController silverstripe-linkfield#348
• API Make AdminRegistrationController a subclass of AdminController silverstripe-mfa#573
• API Make UserDefinedFormAdmin a subclass of FormSchemaController silverstripe-userforms#1348
• API Make HistoryViewerController a subclass of FormSchemaController silverstripe-versioned-admin#369
• API Make ElementalAreaController a subclass of FormSchemaController silverstripe-elemental#1273
• ENH Update code to reflect changes in silverstripe/admin silverstripe-asset-admin#1509
• ENH Update code to reflect changes in silverstripe/admin silverstripe-contentreview#267
• ENH Update code to reflect changes in silverstripe/admin silverstripe-framework#11463
• ENH Update code to reflect changes in silverstripe/admin silverstripe-cms#3024
• FIX Ensure no admin routed links are affected by versioning silverstripe-versioned#481
• DOC Document new FormSchemaController developer-docs#628

[emteknetnz]

@GuySartorelli Thinks are looking good, could you just do a final updated kitchen sink run just to make sure everything integrated correctly

[GuySartorelli]

That's massive overkill considering the scope of the changes since the last run:

• some docs
• some comments
• subsites specifically got rebased on top of some innocuous changes

I'll do it just to avoid ping pong, but it's really not necessary here.