add previous contexts lti sub

make it so that an lti tool can know what the id of a
previous version of the course is

Test Plan:
 * copy a course
 * on the new course, add an lti tool with the substitutions:
   * $Canvas.course.previousContextIds
	 * $Canvas.course.previousCourseIds
 * When you launch the tool in the new course, those should have
   the canvas ids in one, and the lti context_ids in the other

closes PLAT-684

Change-Id: Ieea7a143b3fbb3a164155e1d20d6e54c36e97d22
Reviewed-on: https://gerrit.instructure.com/42887
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>