add variable expansion for getting uuid on root accounts

test plan
 - add a custom variable expansion for $vnd.Canvas.root_account.global_id
  * it should be substituted for root_account.uuid in any LTI launch
 - the capability vnd.Canvas.root_account.global_id should appear in the
   tool consumer profile
 - install an LTI 2 tool that requests the capability
   vnd.Canvas.root_account.global_id
  * launches should come with
    vnd_canvas_root_account_id=<root_account.uuid>

fixes PLAT-2267

Change-Id: I23be140c22c0edd8e20460f648c51cc570a45e80
Reviewed-on: https://gerrit.instructure.com/102077
Tested-by: Jenkins
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brad Humphrey <brad@instructure.com>

lib/lti/capabilities_helper.rb

@@ -12,7 +12,8 @@ module Lti
                                 User.image
                                 Message.documentTarget
                                 Message.locale
-                                Context.id).freeze
+                                Context.id
+                                vnd.Canvas.root_account.uuid).freeze
 
     def self.supported_capabilities
       SUPPORTED_CAPABILITIES

lib/lti/variable_expander.rb

@@ -263,6 +263,15 @@ module Lti
     register_expansion 'Canvas.root_account.id', [],
                        -> { @root_account.id }
 
+    # returns the account uuid for the current context.
+    # @example
+    #   ```
+    #   Ioe3sJPt0KZp9Pw6xAvcHuLCl0z4TvPKP0iIOLbo
+    #   ```
+    register_expansion 'vnd.Canvas.root_account.uuid', [],
+                       -> { @root_account.uuid },
+                       default_name: 'vnd_canvas_root_account_uuid'
+
     # returns the root account sis source id for the current context.
     # @deprecated
     # @example

spec/lib/lti/capabilities_helper_spec.rb

@@ -68,7 +68,8 @@ module Lti
          User.image
          Message.documentTarget
          Message.locale
-         Context.id)
+         Context.id
+         vnd.Canvas.root_account.uuid)
     }
     describe '#supported_capabilities' do
       it 'returns all supported capabilities asociated with launch params' do

spec/lib/lti/variable_expander_spec.rb

@@ -334,6 +334,13 @@ module Lti
         expect(exp_hash[:test]).to eq 54321
       end
 
+      it 'has substitution for $Canvas.account.name' do
+        allow(root_account).to receive(:uuid).and_return('123-123-123-123')
+        exp_hash = {test: '$vnd.Canvas.root_account.uuid'}
+        subject.expand_variables!(exp_hash)
+        expect(exp_hash[:test]).to eq '123-123-123-123'
+      end
+
       it 'has substitution for $Canvas.root_account.sisSourceId' do
         root_account.sis_source_id = 'cd45'
         exp_hash = {test: '$Canvas.root_account.sisSourceId'}