Make org slug changer commit routes.

Without this, the redirect won't be live until something else commits
the routes.  There's no guarantee on when that will happen.
Fix case study export migration.

Publisher class was renamed but migration referenced the old class.
Merge pull request #1876 from alphagov/fix_case_study_migrations

Fix case study export migration.
Extract "exports to publishing api" into mixin.
Add content_id to world locations.
Migration to populate world_location content_ids.
Generic presenter for non-edition placeholders.
Make World Locations publish to publishing API.
Use specific type in placeholder format name.
Send all world locations to content store.
Add world_locations to case_study publisher.
Fix broken tests.
Move concern from models to lib directory
Merge pull request #1871 from alphagov/org_slug_change_commit_routes

Make organisation slug changer commit routes.
Go through presenter finding method in tests

This decouples the logic for deciding the appropriate presenter for a given
model out of the tests.
Rename ExportsToPublishingApi module

"Export" isn't the verb word; "Publish" is more appropriate.
Abstract away calls to the Publishing API

Instead of invoking workers directly when publishing models to the Publishing
API, app code should go through this abstracted interface. This keeps the logic
for deciding how any given model should be represented in the content store
(and how it gets there) in one place.
Merge pull request #1878 from alphagov/world-location-placeholders

Add world locations to content store
Add additional notes around skipping of editions

Once the draft stack is up and running, it is likely that we'll want to push out
draft content too so that it can be previewed.