Enable Person.name.display as an option

Allows for lti launch where you can expand the display name of
a user in lti tools for use.

closes PLAT-2776

Test Plan:
 - Create an lti config that uses Person.name.display
 - Do an lti launch and verify that the expanded value as a param
 - For LTI 2, create a tool proxy that includes the Personl.name.display
  capability
 - Repeat step 2

Change-Id: I7f919bb9af2bb1efb42d8a3bd76cfb3b69378152
Reviewed-on: https://gerrit.instructure.com/134518
Tested-by: Jenkins
Reviewed-by: Weston Dransfield <wdransfield@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Marc Alan Phillips <mphillips@instructure.com>

doc/api/tools_variable_substitutions.md

@@ -550,6 +550,15 @@ Returns the full name of the launching user.
 **Availability**: *when launched by a logged in user*  
 **Launch Parameter**: *lis_person_name_full*  
 
+```
+John Doe
+```
+## Person.name.display
+Returns the display name of the launching user.
+
+**Availability**: *when launched by a logged in user*  
+**Launch Parameter**: *person_name_display*  
+
 ```
 John Doe
 ```

lib/lti/variable_expander.rb

@@ -597,6 +597,17 @@ module Lti
                        USER_GUARD,
                        default_name: 'lis_person_name_full'
 
+    # Returns the display name of the launching user.
+    # @launch_parameter lis_person_name_full
+    # @example
+    #   ```
+    #   John Doe
+    #   ```
+    register_expansion 'Person.name.display', [],
+                       -> { @current_user.short_name },
+                       USER_GUARD,
+                       default_name: 'person_name_display'
+
     # Returns the last name of the launching user.
     # @launch_parameter lis_person_name_family
     # @example

spec/lib/lti/capabilities_helper_spec.rb

@@ -81,6 +81,7 @@ module Lti
          Person.name.given
          Person.name.family
          Person.name.full
+         Person.name.display
          Person.sourcedId
          User.id
          User.image

spec/lib/lti/variable_expander_spec.rb

@@ -174,6 +174,7 @@ module Lti
            Person.name.given
            Person.name.family
            Person.name.full
+           Person.name.display
            Person.sourcedId
            User.id
            User.image
@@ -267,6 +268,11 @@ module Lti
           expanded = variable_expander.enabled_capability_params(enabled_capability)
           expect(expanded.keys).to include 'lis_person_name_full'
         end
+
+        it 'includes Person.name.display when in enabled capability' do
+          expanded = variable_expander.enabled_capability_params(enabled_capability)
+          expect(expanded.keys).to include 'person_name_display'
+        end
       end
     end
 
@@ -966,6 +972,14 @@ module Lti
           expect(exp_hash[:test]).to eq 'Uncle Jake'
         end
 
+        it 'has substitution for $Person.name.display' do
+          user.name = 'Uncle Jake'
+          user.short_name = 'Unc J'
+          exp_hash = {test: '$Person.name.display'}
+          variable_expander.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq 'Unc J'
+        end
+
         it 'has substitution for $Person.name.family' do
           user.name = 'Uncle Jake'
           exp_hash = {test: '$Person.name.family'}