XWIKI-19269: Write a new print service that supports all current features but...

XWIKI-19269: Write a new print service that supports all current features but returns HTML instead of PDF bytes
* Implement a PDF export job that renders a list of documents and collects the results (both XDOM and HTML)
* Implement a PDF Table of Contents macro that uses as input the rendering results of the PDF export job
* Modify the PDF sheet to use the rendering results of the PDF export job when a job id is specified on the request
* Modify the PDF template to use the pdftoc macro when the PDF export job id is specified on the request
* Modify the PDF export UI to trigger the PDF export job and wait for it to finish before showing the print preview modal.