add lti module and module item custom variables

test plan:
 - add a tool with variable substitutions for $Canvas.module.id and $Canvas.moduleItem.id
 - launch the tool as a module item
 - it should substitute the values

Change-Id: I52540a33e4d0ada5a4d8434250e173ff1f9ffdef
Reviewed-on: https://gerrit.instructure.com/44484
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>