save context_id on lti launch

fixes: PS-1538

**test plan
configure and launch lti tool, upon lti tool launch the
lti_context_id for user should be set, and if course launch
the lti_context_id on course object, if account launch, then
lti_context_id on account.  Once these are set, api calls to the
corresponding object can be made using the syntax lti_context_id:id

Change-Id: Icdf02e4f99691be417c024adb2a2751ba2aa9335
Reviewed-on: https://gerrit.instructure.com/35380
Reviewed-by: Brad Humphrey <brad@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Adam Phillipps <adam@instructure.com>
QA-Review: Adam Phillipps <adam@instructure.com>