Race: empty FileGroups can't be resumed

[adamhooper]

Steps to reproduce:

1. Convert some documents
2. Cancel
3. Kill Overview during cancel, after grouped_file_uploads have been deleted for a file group, but before the file_group has been deleted.
4. Restart Overview

Expected results: Overview finishes cleanup upon restarting (or the deletion from grouped_file_uploads was in a transaction, so it gets undone and Overview starts where it left off next time)
Actual results: Overview leaves the file_group alone -- it's a job that stays around forever.

Workaround:

1. Browse to https://www.overviewdocs.com/admin/jobs
2. Inspect the page to see the FileGroup ID (it's part of the table row)
3. Make sure you have the right FileGroup: SELECT * FROM file_group WHERE id = XXXXX
4. Make sure it's empty: SELECT COUNT(*) FROM grouped_file_upload WHERE file_group_id = XXXXX
5. Delete it: change SELECT * to DELETE FROM in the query from step 3