Untangle Publishing API from Panopticon registration

These are not the same things so we shouldn't use the same decorator class,
especially given Panopticon will eventually be retired.
PublishingApiPresenters::Organisation for consistency

Mirrors how we are doing adding editions to content store with Publishing API
Rename publishing API workers

Neither "register" nor "artefact" are terms that are appropriate for the new
publishing API. This renames the workers to better reflect what they are for.

Note that the old files are left in place with the constants set as temporary
aliases of the new classes. This is so that any jobs already queued against the
old worker classes can still be processed. This will need to be cleaned up in a
future commit.
Remove unnecessary queues

We don't set different priorities for these jobs so there is little point in
having a separate queue for them.
Add FIXME note for later cleanup
Merge pull request #1794 from alphagov/publishing-api-presentation

Clean up code that is publishing to Publishing API