Whitelist types of editions that can be indexed

Whilst the `rake rummager:index` task was respecting the searchable
classes list, the `Searchable` mixin wasn't. This moves the list of
searchable classes to a global location in `Whitehall` and uses it to
determine whether to index an `Edition` when saving the item.
Don't index WorldLocationNewsArticle types either

https://www.pivotaltracker.com/story/show/46531711
Make sure DetailedGuides are included in base list
Fix broken `government_search_index` property

It should only include `government_edition_classes`, not
everything. Detailed guides have their own method.

Good catch @chrismdp!
Fix test that I broke

We probably need a better way of handling this than just stubbing things
constantly. Should `GenericEdition` be in the list of allowed classes?
Make World Location News appearance depend on flag

We have a feature flag for WorldLocationNews - we can just use that to
determine searchability of said types.
Merge pull request #362 from alphagov/fix-indexing-edition-types

Fix indexing edition types

https://www.pivotaltracker.com/story/show/46531711
https://www.pivotaltracker.com/story/show/46468431