Moved session from query string to context template

This PR reflects the concept of "context template" I'd mentioned in prior conversations about launch, and I've moved session_id out of the query string to use that template. This keeps the query string down to the minimum # of parameters necessary to read state, which is I thought what we had discussed (please correct me if I'm wrong).

What this "context template" concept does is provide a way for the LMS to push context information to AUs that should be included in statements. This is the only way for an LMS to add this additional context, since an LRS is not permitted to alter statements it receives.

While doing that I removed the description "Launch parameter data specific to the AU from the course structure." It seemed like the context template would cover anything that should be written back in statements, and anything that wouldn't would need to be specified specifically to be useful -- so I think we're not done with defining the contents of this document, but there is no purpose in generic language about what can go in there.