Refactor admin/* routing logic from LeftAndMain into its own abstract class

[GuySartorelli]

We often subclass LeftAndMain just to get controllers routed in the admin section, and then hide them because they're not actually left and main sections.

We should instead have an abstract controller class we can use as the superclass for those which doesn't include all of the complexity of LeftAndMain.

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.
  • I'm going with AdminController for now, as it's less likely to be confused with AdminRootController.

Acceptance criteria

• A new abstract AdminRouteController class is created
• AdminRootController checks for (non-abstract) subclasses of the new class instead of subclasses of LeftAndMain to set up routing rules
• Methods, properties, and configuration required for routing or permissions are moved from LeftAndMain to the new class (see this comment for recommendations about this)
• LeftAndMain subclasses the new class
• Any controller in supported modules that currently subclasses LeftAndMain but only needs the routing and permissions logic is made to subclass the new class instead.
• Any controller in supported modules that uses an admin route but which doesn't currently subclass LeftAndMain is made a subclass of the new class instead.
• Documentation is updated to indicate best practice for controllers using a /admin/ route

PRs

kitchen sink run
Compare with CI run in silverstripe/recipe-kitchen-sink

• NEW AdminController as superclass for /admin/* routed controllers #1836
• API Make use of the new AdminController silverstripe-externallinks#151
• API Update method signature to match parent class silverstripe-asset-admin#1502
• API Update method signature to match parent class silverstripe-elemental#1262
• API Update method signature to match parent class tractorcow-farm/silverstripe-fluent#896
• API Update method signature to match parent class silverstripe-frameworktest#207
• API Update method signature to match parent class silverstripe-campaign-admin#335
• API Update method signature to match parent class silverstripe-versioned-admin#366
• API Update method signature to match parent class silverstripe-cms#3015
• ENH Use config instead of runtime code to remove menu item silverstripe-linkfield#342
• ENH Use config instead of runtime code to remove menu item silverstripe-userforms#1337
• API Improve type safety for Controller::join_links() silverstripe-framework#11426
• DOC Document new AdminController developer-docs#598

[GuySartorelli]

Unfortunately most classes that subclass LeftAndMain but aren't left or main require form schema functionality. But this paves the way to liberate them.
See #1762 for the next stage in the process.