add lti variable subs for section ids

Now an LTI launch can list the section ids that a user has
enrollments in with: $Canvas.course.sectionIds and
$Canvas.course.sectionSisSourceIds

Test Plan:
 * Create a user with enrollments in a section and multiple sections
 * Configure an LTI tool to have the 2 new section variable subs
 * The ids/sis ids of the section that the user has a direct
   enrollment in should be substituted
 * If a teacher is only enrolled in the main section all the other ones
   will _not_ be listed.

closes PLAT-784

Change-Id: I0af6b674a710767f4f38693f86fac8b988a21f06
Reviewed-on: https://gerrit.instructure.com/45462
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>