Add world locations to content store

Add content_ids to world_locations, export them to content store, and add a world_locations key to the case_study links hash.

To enable this, I've extracted out the "exports to publishing API" for non-Edition models into a new mixin, which now takes care of setting and validating a content_id and exporting on create/update (note this now happens on commit, rather than on write, to avoid possible transaction issues in sidekiq).

Additionally, this renames the organisation presenter to a generic placeholder presenter, which uses polymorphic_path to generate the base path and uses format "placeholder_#{contenttype}".