Directly associate editions with topics

This commit also moves the code in Edition::Topics into Edition, as
simply including Edition::Topics in Edition doesn't insert
Edition::Topics into Edition's ancestors list in front of
Searchable::Mixin, which prevents us from defining a #search_index
method that will be accessible via chained calls to super.
Reformat long lines for legibility
Add rake task to copy policies' topics to related editions
Replace KEY_MAPPING variable with ternary operator