Improve behavior of statement download buttons

* Download buttons are disabled with (more) helpful message in cases
  where the cert has been earned but no PDF has been generated yet.
* Additional text on the course home page in the Congratulations! box.
* Reinstate chat iframe assuming #2427 lands before this does.
* Minor code cleanups in various places (removing unused imports,
  removing debug prints)
* issue_certificate_single uses same routine to get output filename that
  the template calls to get it, so they vary together from now on.
* (closes #2430)
Statement profile button update

* Adds "Check back soon!"
Merge pull request #2433 from Stanford-Online/jrbl-2430-smart-cert-download-buttons

Improve behavior of statement download buttons