Adjust variable substitution for membership service

fixes PLAT-1450

Test plan:
* Using the test tool https://lti-tool-provider-example.herokuapp.com/
  set up some XML for a tool that has a 'collaboration' placement and
  custom variable with the following variable substitution
    $Canvas.api.membershipServiceUrl
* Ensure that on a course collaboration launch the membership service
  url is of the following form
    /api/lti/courses/:course_id/membership_service
* Ensure that on an account group collaboration or course group
  collaboration launch the membership service url is of the following
  form
    /api/lti/groups/:group_id/membership_service

Change-Id: I38ce63ba0d95a12963e99251c2abf660573e9c7b
Reviewed-on: https://gerrit.instructure.com/78276
Reviewed-by: Nathan Mills <nathanm@instructure.com>
Tested-by: Jenkins
QA-Review: August Thornton <august@instructure.com>
Product-Review: Andrew Butterfield <abutterfield@instructure.com>

lib/lti/variable_expander.rb

@@ -106,8 +106,8 @@ module Lti
                        -> { "#{@request.scheme}://#{HostUrl.context_host(@root_account, @request.host)}" }
 
     register_expansion 'Canvas.api.membershipServiceUrl', [],
-                       -> { @controller.course_membership_service_url(@context) },
-                       COURSE_GUARD
+                       -> { @controller.polymorphic_url([@context, :membership_service]) },
+                       -> { @context.is_a?(Course) || @context.is_a?(Group) }
 
     # returns the account id for the current context.
     # @example

spec/lib/lti/variable_expander_spec.rb

@@ -22,6 +22,8 @@ module Lti
     let(:root_account) { Account.new }
     let(:account) { Account.new(root_account: root_account) }
     let(:course) { Course.new(account: account) }
+    let(:group_category) { course.group_categories.new(name: 'Category') }
+    let(:group) { course.groups.new(name: 'Group', group_category: group_category) }
     let(:user) { User.new }
     let(:assignment) { Assignment.new }
     let(:substitution_helper) { stub_everything }
@@ -206,13 +208,25 @@ module Lti
         expect(exp_hash[:test]).to eq Shard.current.id
       end
 
+      context 'context is a group' do
+        subject { described_class.new(root_account, group, controller, current_user: user) }
+
+        it 'has substitution for $Canvas.api.membershipServiceUrl when context is a group' do
+          exp_hash = { test: '$Canvas.api.membershipServiceUrl' }
+          group.stubs(:id).returns('1')
+          controller.stubs(:polymorphic_url).returns("/api/lti/groups/#{group.id}/membership_service")
+          subject.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq "/api/lti/groups/1/membership_service"
+        end
+      end
+
       context 'context is a course' do
         subject { described_class.new(root_account, course, controller, current_user: user) }
 
-        it 'has substitution for $Canvas.api.membershipServiceUrl' do
+        it 'has substitution for $Canvas.api.membershipServiceUrl when context is a course' do
           exp_hash = { test: '$Canvas.api.membershipServiceUrl' }
           course.stubs(:id).returns('1')
-          controller.stubs(:course_membership_service_url).returns("/api/lti/courses/#{course.id}/membership_service")
+          controller.stubs(:polymorphic_url).returns("/api/lti/courses/#{course.id}/membership_service")
           subject.expand_variables!(exp_hash)
           expect(exp_hash[:test]).to eq "/api/lti/courses/1/membership_service"
         end