Fix indexing edition types

When running `rake rummager:index` the `government_search_index` constructed a list of `Edition` derived classes that are allowed to be indexed. This behaviour wasn't pushed out into the `Searchable` mixin, so any `Editions` edited that are on the banned list are indexed regardless of the `government_search_index` behaviour.

This fixes that, and in doing so fixes two bugs:
https://www.pivotaltracker.com/story/show/46531711
https://www.pivotaltracker.com/story/show/46468431