Rename `publishingApp` struct and make it public for easier reuse

This will be used for the content-store request/response struct as
well. To make this easier we need to make it a public struct.

Also, making it public won't be enough as there's an internal field
called `PublishingApp` already which will lead to issues when trying
to get at the underlying value, hence the rename to
`PublishingApplication`.
Wire up requests to publish in the content-store to register URLs

This is a test which uses the top-level HTTP muxer for the application
and makes sure that we're taking the `publishing_app` out and
registering the path with the url-arbiter application.

This is the beginning of the end-to-end testing for the application in
question.