Refactor CMSMain and its subclasses

[sabina-talipova]

Description

See:

silverstripe-cms/code/Controllers/CMSMain.php

Line 83 in 58a2d58

* @todo Create some base classes to contain the generic functionality that will be replicated.

This "TODO" comment was created in 2016. We need to investigate this matter and assess its current relevance.

CMSMain and its subclasses are a mess. We should look at refactoring these, preferably in a generic way that provides a more generic "TreeModelAdmin" style class. The CMSMain could just be a subclass of that.
We should ideally not need a sub CMSPageAddController, CMSPageEditController, CMSPageSettingsController, CMSPagesontroller, etc.

[GuySartorelli]

Closing as a dupe of silverstripe/.github#257