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
• 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