Demo course about pages were not available (and probably many other
pages as well) because the preview url was the same as the real url.
Kudos to @dannielarriola for solving this!
Close issues #11#7.
By using variables in the nginx configuration to point to lms/cms, we
allow nginx not to require that lms and cms are up to run. This is
convenient for debugging production settings: just launch an nginx, then
runserver in an lms.
