Support Context.id.history LTI subsitution variable

Install an LTI App Store.  Create a site with at least Gradebook and Lessons.  Install and LTI tool in that site - perhaps Trophy.  Launch the tool with debug and search for the context_id_history parameter - it should be "empty" which means it contains "$Context.id.history" (i.e. not a number). Then make a second site, with Lessons and Gradebook but also import from the first site.  At the "Reuse Material from Other Sites" check them all and continue.  Then launch the tool and look for context_id_history - it should be something like "989d0991-b54d-46cd-95e1-14486eb194db" - the site ID of the site you copied from. Then make a third site, with Lessons and Gradebook but also import from the second site.  At the "Reuse Material from Other Sites" check them all and continue.  Then launch the tool and look for context_id_history - it should be something like "989d0991-b54d-46cd-95e1-14486eb194db,e47716ed-bbc0-43e5-bc65-9529db2a016b" - the site IDs of the first and second site.