Adding some comments to simpletool.views.lti_launch

lti_launch redirects to main_page, which loses the POST vars.
main_page is trying to display the POST vars.

Added comments to explain what that view should probably be doing:
-[x] setting a session up using the lti post vars
-[x] redirecting to main_page

Added a comment with a return render(...) that will allow the main_page template to have the post vars.