Add support for expanding the Course Integration ID

When performing a LTI launch this allows the course
integration ID be expanded in the custom data that is made available in an LTI launch.

Closes INTEROP-6480
Closes GH-1606

flag = none
test plan:
 - specs pass

Change-Id: I5a21eab4f02f5da1f45f714d265903d5615bce6c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/256921
Reviewed-by: Weston Dransfield <wdransfield@instructure.com>
QA-Review: Weston Dransfield <wdransfield@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Product-Review: Alex Slaughter <aslaughter@instructure.com>

doc/api/tools_variable_substitutions.md

@@ -550,6 +550,15 @@ returns the current course sis source id.
 **Availability**: *when launched in a course*
 
 
+```
+1234
+```
+## com.instructure.Course.integrationId
+returns the current course integration id.
+
+**Availability**: *when launched in a course*
+
+
 ```
 1234
 ```

lib/lti/variable_expander.rb

@@ -576,6 +576,15 @@ module Lti
                        -> { @context.sis_source_id },
                        COURSE_GUARD
 
+    # returns the current course integration id.
+    # @example
+    #   ```
+    #   1234
+    #   ```
+    register_expansion 'com.instructure.Course.integrationId', [],
+                       -> { @context.integration_id },
+                       COURSE_GUARD
+
     # returns the current course start date.
     # @example
     #   ```

spec/lib/lti/variable_expander_spec.rb

@@ -931,6 +931,13 @@ module Lti
           expect(exp_hash[:test]).to eq 'course1'
         end
 
+        it 'has substitution for $com.instructure.Course.integrationId' do
+          course.integration_id = 'integration1'
+          exp_hash = {test: '$com.instructure.Course.integrationId'}
+          variable_expander.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq 'integration1'
+        end
+
         it 'has substitution for $Canvas.enrollment.enrollmentState' do
           allow(Lti::SubstitutionsHelper).to receive(:new).and_return(substitution_helper)
           allow(substitution_helper).to receive(:enrollment_state).and_return('active')