Add a page to check the status of upload transactions

Currently when running in celery's asynch mode, we redirect directly to the draft builder page.  This adds an intermediate page that checks the transaction status before redirecting.  In this way, we can also redirect back to an error page on failure.

The status check implementation is a bit quick and dirty for the moment.  In future, we can make this more robust.  There are outstanding bugs when displaying the source edit form on failure, and the support pipeline needs some work, but these will be separate issues.