Make LeftAndMain abstract and other small refactoring in this class

[GuySartorelli]

There's no good reason to instantiate LeftAndMain. It should be abstract.

Related

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

Acceptance criteria

• LeftAndMain is made abstract
• Anything that requires instantiating LeftAndMain is refactored to use a concrete class instead.
• A new abstract getModelClass() method is created
• The tree_class configuration property is removed - anything that used this configuration is updated to call getModelClass() instead.
• Methods which used that config are updated according to the "tree_class config" section in this comment unless doing so causes unforeseen problems
• Recommendations in the "Additional recommendations" section in this comment are applied unless doing so causes unforeseen problems