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.
Make Publishing API presenters locale aware

The presenters present the instances appropriately for the current locale.
PublishingApiWorker can publish specific locales

Now that the presenters are locale-aware, the worker can push a content item to
the Publishing API in a given locale, defaulting to the default locale
otherwise.
Publish all translations to the Publishing API

Whitehall::PublishingApi.publish and Whitehall::PublishingApi.republish now send
all available translations to the Publishing API.

Note that these methods will now blow up if handed a non-translatable model, but
as we are not yet sending any non-translatable models to the Publishing API,
it's impossible to write a test to support this behaviour. That can be done at
the appropriate time.
Data migration to republish all translated things
Merge pull request #1879 from alphagov/push-translations-to-publishing-api

Push translations to publishing api
Set test expectation on PublishingApi.republish.
Merge pull request #1887 from alphagov/fix-topic-changes-test

Fix topic changes test.