add $Canvas.masqueradingUser.id substitutuion

test plan:
 - add a tool with the custom variable $Canvas.masqueradingUser.id
 - masquerade as a user
 - launch the tool
 - $Canvas.masqueradingUser.id should be your actual canvas id

fixes PLAT-657

Change-Id: I5c6cf1532cac7fa4dcf065652d87500e5d30e474
Reviewed-on: https://gerrit.instructure.com/41750
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>