Refactor logic for sections that deal with only a single record out of LeftAndMain into its own class

[GuySartorelli]

Most subclasses of LeftAndMain deal with more than one record at a time - be it ModelAdmin which handles multiple records in a GridField, or CMSMain which deals with multiple records in a hierarchical structure.

The logic in LeftAndMain which is intended for sections that only edit a single record should be abstracted out into its own abstract class.

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.
• Refer to SPIKE Analyse how LeftAndMain and CMSMain are put together and how they could work better silverstripe-cms#2933 (comment) for recommendations on some implementation details

Acceptance criteria

• A new abstract SingleRecordAdmin class is created, which is a subclass of LeftAndMain
• AdminRootController checks for (non-abstract) subclasses of the new class instead of subclasses of LeftAndMain to set up routing rules - already handled by Refactor admin/* routing logic from LeftAndMain into its own abstract class #1761
• Methods, properties, and configuration used only for editing single records are moved from LeftAndMain to the new class
• CMSProfileController, SiteConfigLeftAndMain, and any other controllers in supported modules which only edit a single record are made subclasses of this new class
• Functionality that's reimplemented in CMSProfileController, SiteConfigLeftAndMain, etc are made generic and are implemented in the new class directly

PRs

CMS 5 PRs

• API Deprecate API that will be renamed #1844
• API Deprecate API that will be renamed silverstripe-siteconfig#181
• API Deprecate API that will be renamed silverstripe-campaign-admin#339
• API Deprecate API that will be renamed silverstripe-reports#201
• API Deprecate API that will be renamed silverstripe-asset-admin#1506
• API Deprecate API that will be renamed silverstripe-cms#3021
• DOC Document deprecation of some API developer-docs#616

CMS 6 PRs

kitchen sink ci

• NEW SingleRecordAdmin class for editing one record at a time #1842
• API Use the new SingleRecordAdmin class silverstripe-siteconfig#179
• API Update code to reflect changes in silverstripe/admin silverstripe-cms#3019
• API Update code to reflect changes in silverstripe/admin silverstripe-subsites#606
• API Update code to reflect changes in silverstripe/admin silverstripe-campaign-admin#337
• API Update code to reflect changes in silverstripe/admin silverstripe-reports#200
• API Update code to reflect changes in silverstripe/admin silverstripe-asset-admin#1504
• API Update code to reflect changes in silverstripe/admin tractorcow-farm/silverstripe-fluent#902
• DOC Document new SingleRecordAdmin class developer-docs#609

[emteknetnz]

@GuySartorelli even though it's draft, I've started by reviewing the CMS 6 framework PR first as that could change the CMS 5 PRs