logged-out users on public courses get redirected to preview
minor changes
logged-in users that are not course members get preview
Fix for 404 when empty JABBER_DOMAIN set

* Properly handle empty (rather than merely missing) JABBER_DOMAIN
  configuration variable.
merge conflict resolved
Merge branch 'master' of github.com:Stanford-Online/class2go into jrbl-empty-tps-report-domain-fix

Conflicts:
	main/courses/views.py
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!"
Remove confusing config from database_example.py

* Having sample chat config in database_example is a little confusing
  for people, so let's just not put it in there until we're sure
  everything works on our setup.
Merge pull request #2433 from Stanford-Online/jrbl-2430-smart-cert-download-buttons

Improve behavior of statement download buttons
Merge pull request #2427 from Stanford-Online/jrbl-empty-tps-report-domain-fix

Fix for 404 when empty JABBER_DOMAIN set
Merge pull request #2394 from Stanford-Online/jbau-redirect-to-preview

Jbau redirect to preview
fixing merge conflict with master