Add com.instructure.User.student_view variable expansion

At New Quizzes Item Analysis the submissions
with the student view should be excluded. For other LTI tools
it is a useful info that the tool is launched by a student
view `fake student`.

closes: QUIZ-12877
flag=none

test plan:
- have an LTI tool tha could be launched during test steps
- add `canvas_user_student_view=$com.instructure.User.student_view`
  as custom lti parameter

- launch tool with a student user
- ensure `custom_canvas_user_student_view` is false

- launch tool with an admin user
ensure `custom_canvas_user_student_view` is false

- launch tool with from a student view
- ensure `custom_canvas_user_student_view` is true

Change-Id: If86ac6452d01045fc85331aaaf42052a5319f512
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/334992
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Product-Review: Alexis Nast <alexis.nast@instructure.com>
Reviewed-by: Paul Gray <paul.gray@instructure.com>
QA-Review: Rajmund Csehil <rajmund.csehil@instructure.com>

doc/api/tools_variable_substitutions.md

@@ -250,6 +250,13 @@ Returns the host of the rich content service for the current region.
 
 ```
 "rich-content-iad.inscloudgate.net"
+```
+## com.instructure.User.student_view
+Returns true if the user is launching from student view.
+
+**Availability**: *always*  
+**Launch Parameter**: *com_instructure_user_student_view*  
+
 ```
 ## com.instructure.Observee.sisIds
 returns all observee ids linked to this observer as an String separated by `,`.

lib/lti/variable_expander.rb

@@ -317,6 +317,19 @@ module Lti
                        },
                        default_name: "com_instructure_rcs_app_host"
 
+    # Returns true if the User is from student view process
+    #
+    # @example
+    #   ```
+    #  "true"
+    #  "false"
+    #   ```
+    register_expansion "com.instructure.User.student_view",
+                       [],
+                       -> { @current_user.fake_student? || false },
+                       USER_GUARD,
+                       default_name: "com_instructure_user_student_view"
+
     # Returns the RCS Service JWT for the current user
     #
     # @internal

spec/lib/lti/variable_expander_spec.rb

@@ -1337,6 +1337,30 @@ module Lti
           end
         end
 
+        describe "$com.instructure.User.student_view" do
+          subject do
+            variable_expander.expand_variables!({ test: "$com.instructure.User.student_view" })[:test]
+          end
+
+          context "user is not logged in" do
+            let(:user) { nil }
+
+            it { is_expected.to eq "$com.instructure.User.student_view" }
+          end
+
+          context "user is generated by student view" do
+            before { allow_any_instance_of(User).to receive(:fake_student?).and_return(true) }
+
+            it { is_expected.to be true }
+          end
+
+          context "user is not from student view" do
+            before { allow_any_instance_of(User).to receive(:fake_student?).and_return(nil) }
+
+            it { is_expected.to be false }
+          end
+        end
+
         describe "$com.instructure.instui_nav" do
           subject do
             variable_expander.expand_variables!({ test: "$com.instructure.instui_nav" })[:test]