Added "filename" attribute and database column to the OpmlExportJobState model.

This attribute will hold the filename of the generated OPML file. It will only take value if the state of the job is "SUCCESS".
The updating of the job state to "SUCCESS" now is done in OPMLExporter, instead of in ExportSubscriptionsJob.

Also the name of the file sent attached to the notification email is always "feedbunch_export.opml", regardless of the internal name with which the file is saved.
Added callback that delestes any associated OPML file before destroying an OpmlExportJobState.

This means that OPMLExporter.export no longer needs to explicitly delete old OPML files when doing a new OPML export. This is automatically taken care of by User#export_subscriptions, when it creates a new opml_export_job_state and deletes the old one (which triggers the deletion of the old OPML file, if any).