User no longer needs to explicitly delete associated OPML files when an instance is destroyed.

What happens is: when the User instance is destroyed, its associated OpmlExportJobState is destroyed; and this triggers the deletion of the OPML file, if any. This is taken care of by ActiveRecord callbacks.
Moved the code that uploads the OPML file to Amazon S3 and updates the job state from OPMLExporter to ExportSubscriptionsJob.

I think this is a better separation of concerns:

- OPMLExporter.export only creates the OPML document.
- The state attribute of the associated OpmlExportJobState is updated by the job itself (the ExportSubscriptionsJob class).