Visual header, body, and footer composition screen.

* Drag-and-drop section ordering with automatic saving.
* Add existing reusable documents to regions.
* Remove placements without deleting documents.
* Create independent page-specific copies.
* Shared-document usage warnings.
* Document creation, duplication, deletion protection and filtering.
* Draft and published document states.
* Document and placement scheduling.
* Authenticated homepage preview.
* Draft sections displayed in preview with a clear draft marker.
* Content Composer added to the administration sidebar.
* Public rendering now respects:
    * Publication status
    * Future publication dates
    * Expiry dates
    * Placement start and end dates
    * Enabled state
* Original Saasland homepage remains as the fallback when no composition exists.

This commit is contained in: