add variable substitution for masquerading user's LTI id

fixes PLAT-976

test plan:
- create an external app within a course
- add MasqUserID=$Canvas.masqueradingUser.userId as a custom field
- make sure the value of MasqUserID is set to the logged in user's LTI id
- masquerade as a user who has access to the external app
- make sure the value of MasqUserID is set to the lagged in user's LTI id, not the id of the user being impersonated

Change-Id: I80bd6dbf38d66937b86e1f636db2fafc219c6ef7
Reviewed-on: https://gerrit.instructure.com/52929
Tested-by: Jenkins
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Weston Dransfield <wdransfield@instructure.com>

lib/lti/variable_expander.rb

@@ -233,6 +233,10 @@ module Lti
                        -> { @current_pseudonym.id },
                        -> { @current_pseudonym != @current_user }
 
+    register_expansion 'Canvas.masqueradingUser.userId', [],
+                       -> { @tool.opaque_identifier_for(@controller.logged_in_user) },
+                       USER_GUARD
+
     register_expansion 'Canvas.xapi.url', [],
                        -> { @controller.lti_xapi_url(Lti::AnalyticsService.create_token(@tool, @current_user, @context)) },
                        -> { @current_user && @context.is_a?(Course) && @tool }

spec/lib/lti/variable_expander_spec.rb

@@ -34,6 +34,7 @@ module Lti
       shard_mock = mock('shard')
       shard_mock.stubs(:settings).returns({encription_key: 'abc'})
       m.stubs(:shard).returns(shard_mock)
+      m.stubs(:opaque_identifier_for).returns("6cd2e0d65bd5aef3b5ee56a64bdcd595e447bc8f")
       m
     end
     let(:controller) do
@@ -546,6 +547,12 @@ module Lti
           expect(exp_hash[:test]).to eq 7878
         end
 
+        it 'has substitution for $Canvas.masqueradingUser.userId' do
+          exp_hash = {test: '$Canvas.masqueradingUser.userId'}
+          subject.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq '6cd2e0d65bd5aef3b5ee56a64bdcd595e447bc8f'
+        end
+
         it 'has substitution for Canvas.module.id' do
           content_tag = mock('content_tag')
           content_tag.stubs(:context_module_id).returns('foo')