Make URLs properly hiearchical

Currently URLs for lessons expose the file directories used internally in the filesystem, rather than matching the sessions in the breadcrumbs. Observe:

http://naucse.python.cz/2017/pyladies-brno-jaro-po/
http://naucse.python.cz/2017/pyladies-brno-jaro-po/sessions/install/ (issue pyvec/naucse.python.cz#164)
http://naucse.python.cz/2017/pyladies-brno-jaro-po/beginners/cmdline/
http://naucse.python.cz/2017/pyladies-brno-jaro-po/git/install/

The last two URLs are a part of the "install" session. In order to match the breadcrumbs, they should instead say something like:

http://naucse.python.cz/2017/pyladies-brno-jaro-po/install/cmdline/
http://naucse.python.cz/2017/pyladies-brno-jaro-po/install/git/

Additionally, this issue enables you to "cheat" and access pages that are not truly a part of the course, like http://naucse.python.cz/2017/pyladies-brno-jaro-po/intro/flask/. This only works with a real webserver and not Elsa, however, since these pages are not linked anywhere.

Blocked by pyvec/naucse#5, because any URL scheme changes require redirects.