Add page-builder models: Theme, Page, PageSection, PageBlock, PageLink, and duplication job

Add page-builder models: Theme, Page, PageSection, PageBlock, PageLink, and duplication job: a task in bird-critic-1.5-full-300 (Harbor dataset). Spree needs a page-builder system backed by persistent models. A Spree::Theme belongs to a store and acts as a top-level container; a Spree::Page…

The task

Spree needs a page-builder system backed by persistent models. A `Spree::Theme` belongs to a store and acts as a top-level container; a `Spree::Page` belongs to a theme or store (polymorphic `pageable`); `Spree::PageSection` belongs to a page or theme; `Spree::PageBlock` belongs to a section; and `Spree::PageLink`…

Part of kochsnow/bird-critic-1.5-full-300.