PDF and XML WF: only notify when preview is generated *if* review is required
Currently we send an email notification to the user who uploaded the file when that file generates a preview. In the Word WF this makes sense because the user is always expected to check the preview.
In the XML and PDF workflow, no one needs to check a preview unless the book has a review workflow (ie. the setting "require Previewers approval before publishing" is turned ON). Books that have this setting turned OFF get automatically released (see #435 (closed)).
Therefore:
- When book has setting "require Previewers approval before publishing" ON, send notification to user who uploaded the file.
- When book has setting "require Previewers approval before publishing" OFF, do not send notification.