Invalid: Feature/lti

This PR adds LTI so that the tool can work in canvas. 

To setup in Canvas as a module served from localhost, add an entry to `/etc/hosts`:

- 127.0.0.1   localhost.harvard.edu

Then go to the modules page of canvas and create an *external tool* with the following url:

- http://localhost.havard.edu:8000/lti-launch