add $Canvas.course.endAt LTI variable substitution

closes PLAT-5288
flag=none

Test plan:
* create a course with an end_at date
* set up a tool with the "$Canvas.course.endAt" LTI variable
  substitution
* launch the tool and look at the LTI parameters in dev tools
  and see that the variable is set correctly
* remove the end_at date, relaunch the tool, check that the variable
  is now an emptry string

Change-Id: Ifba70ebd6cb9fe8d39da1d24fb6ae69e97902828
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/223702
Reviewed-by: Marc Phillips <mphillips@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Marc Phillips <mphillips@instructure.com>
Product-Review: Evan Battaglia <ebattaglia@instructure.com>

doc/api/tools_variable_substitutions.md

@@ -513,6 +513,15 @@ returns the current course start date.
 ```
 2018-01-15 00:00:00 -0700
 ```
+## Canvas.course.endAt
+returns the current course end date.
+
+**Availability**: *when launched in a course*
+
+
+```
+2019-05-10 00:00:00 -0600
+```
 ## Canvas.course.workflowState
 returns the current course workflow state. Workflow states of "claimed" or "created"
 indicate an unpublished course.

lib/lti/variable_expander.rb

@@ -518,6 +518,15 @@ module Lti
                        -> { @context.start_at },
                        COURSE_GUARD
 
+    # returns the current course end date.
+    # @example
+    #   ```
+    #   2018-01-15 00:00:00 -0700
+    #   ```
+    register_expansion 'Canvas.course.endAt', [],
+                       -> { @context.end_at },
+                       COURSE_GUARD
+
     # returns the current course workflow state. Workflow states of "claimed" or "created"
     # indicate an unpublished course.
     # @example

spec/lib/lti/variable_expander_spec.rb

@@ -1106,6 +1106,14 @@ module Lti
           expect(exp_hash[:test]).to eq '2015-05-21 17:01:36'
         end
 
+        it 'has substitution for $Canvas.course.endAt' do
+          course.conclude_at = '2019-04-21 17:01:36'
+          course.save!
+          exp_hash = {test: '$Canvas.course.endAt'}
+          variable_expander.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq '2019-04-21 17:01:36'
+        end
+
         it 'has a functioning guard for $Canvas.term.name when term.name is not set' do
           term = course.enrollment_term
           exp_hash = {test: '$Canvas.term.name'}