Clean up AdminDocumentProducerController (#2620)

A few changes to bring this controller in line with the other admin UI
controllers:

* Remove its copy of the RedirectAttributes extension properties for setting
  success and failure messages. Those extensions are already available as
  top-level functions.

* Don't require its URL to have a trailing slash, which means no need for
  the auto-redirect to add a trailing slash. All the form target URLs in
  its HTML page are full paths, so there's no issue with relative paths
  needing the trailing slash to resolve properly.

* Simplify the redirect function and rename it to match the names of the
  redirect functions in other controllers.