Publish documents only if all users have accepted the license

The original code for checking the state of license acceptance was

```
SELECT bool_and(accepted)
FROM
  pending_documents AS pd,
  license_acceptances AS la
WHERE
  pd.id = %s
  AND
  pd.uuid = la.uuid
```

which returns true if accepted is true and null.  This means cnx-publishing was
publishing content even if some users have not accepted the license.

Close #68