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.