Add search_format_types to the editions searchable interface

This is an array of values that allows us to tag documents as their type and any grouped types that might cut across multiple classes (e.g. StatisticalDataSets and Publications with a PublicationType that is in PublicationType.statistical are both tagged in search_format_types with publicationesque-statistics).
Editions can generate their own search_format_type programatically

Most instance methods use the class method and add it to the result from the superclasses.
Fill in the blanks in the rummager implementation of DocumentFilter

We make use of the search_format_types to do filtering instead of a combination of format and the various xxx_type fields.