Add a progress bar to the PDFAttachmentReporter
Add a progress bar to the PDFAttachmentReporter

- Gives an indication of how many more attachments are left to process,
  as this can sometimes take a few hours.
- Does require doing the query up front in order to work out how many
  rows need to be processed in advance.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall