Improve behavior of statement download buttons

@dcadams, please. #2427 should land first though.
@jinpa and @caesar2164, please confirm that this looks ok (screenshots attached below)

* 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)

![homepage-trimmed](https://f.cloud.github.com/assets/105487/300713/71e351d8-95a2-11e2-9acb-9119fe079cd6.png)
![profile-trimmed](https://f.cloud.github.com/assets/105487/300715/765edd0e-95a2-11e2-931f-8bb76b0485ed.png)