Refactor tree filtering in CMSMain/HierarchyModelAdmin

[GuySartorelli]

Form fields for filtering in CMSMain are hardcoded - and some of the functionality is duplicated or else needlessly different between this filter functionality and GridFieldFilterHeader.

Relevant

• SPIKE Analyse how LeftAndMain and CMSMain are put together and how they could work better #2933
• original POC PR - this PR includes many additional notes and implementation details

Acceptance Criteria

• The fields to use for filtering are pulled from the model in the same way GridFieldFilterHeader does (i.e. it uses searchableFields())
• SearchContext is used for filtering in CMSMain the same way it is in GridFieldFilterHeader
• Any CMSSiteTreeFilter logic is either made generic if possible, or else used by the SearchContext for SiteTree wherever filtering pages is used
  • i.e. filtering pages in a gridfield uses the same logic as filtering pages in CMSMain
• A shared template and shared javascript/css are used for the filter in GridFieldFilter and CMSMain (with the ability to use different custom templates as desired - e.g. by trying to get specific templates by class name first, and falling back to the shared one)
• As much as feasible, PHP logic for filtering in GridFieldFilterHeader and CMSMain is shared