Refactor API noteable types constants

This replaces API::Notes::NOTEABLE_TYPES and
API::Discussions::NOTEABLE_TYPES with class methods defined in a
separate helper modules. This allows EE to extend the list of noteable
types, without having to modify the constant in-place. We can't define
these methods directly in the API classes, as they would be used before
we're able to extend them in EE.

ee/lib/ee/api/helpers/discussions_helpers.rb

+# frozen_string_literal: true
+
+module EE
+  module API
+    module Helpers
+      module DiscussionsHelpers
+        extend ActiveSupport::Concern
+
+        class_methods do
+          extend ::Gitlab::Utils::Override
+
+          override :noteable_types
+          def noteable_types
+            [::Epic, *super]
+          end
+        end
+      end
+    end
+  end
+end

ee/lib/ee/api/helpers/notes_helpers.rb

@@ -4,6 +4,17 @@ module EE
   module API
     module Helpers
       module NotesHelpers
+        extend ActiveSupport::Concern
+
+        class_methods do
+          extend ::Gitlab::Utils::Override
+
+          override :noteable_types
+          def noteable_types
+            [::Epic, *super]
+          end
+        end
+
         def find_group_epic(id)
           finder_params = { group_id: user_group.id }
           EpicsFinder.new(current_user, finder_params).find(id)

lib/api/discussions.rb

@@ -7,9 +7,7 @@ module API
 
     before { authenticate! }
 
-    NOTEABLE_TYPES = [Issue, Snippet, Epic, MergeRequest, Commit].freeze
-
-    NOTEABLE_TYPES.each do |noteable_type|
+    Helpers::DiscussionsHelpers.noteable_types.each do |noteable_type|
       parent_type = noteable_type.parent_class.to_s.underscore
       noteables_str = noteable_type.to_s.underscore.pluralize
       noteables_path = noteable_type == Commit ? "repository/#{noteables_str}" : noteables_str

lib/api/helpers/discussions_helpers.rb

+# frozen_string_literal: true
+
+module API
+  module Helpers
+    module DiscussionsHelpers
+      def self.noteable_types
+        # This is a method instead of a constant, allowing EE to more easily
+        # extend it.
+        [Issue, Snippet, MergeRequest, Commit]
+      end
+    end
+  end
+end
+
+API::Helpers::DiscussionsHelpers.prepend(EE::API::Helpers::DiscussionsHelpers)

lib/api/helpers/notes_helpers.rb

@@ -3,7 +3,11 @@
 module API
   module Helpers
     module NotesHelpers
-      prepend EE::API::Helpers::NotesHelpers # rubocop: disable Cop/InjectEnterpriseEditionModule
+      def self.noteable_types
+        # This is a method instead of a constant, allowing EE to more easily
+        # extend it.
+        [Issue, MergeRequest, Snippet]
+      end
 
       def update_note(noteable, note_id)
         note = noteable.notes.find(params[:note_id])
@@ -113,3 +117,5 @@ module API
     end
   end
 end
+
+API::Helpers::NotesHelpers.prepend(EE::API::Helpers::NotesHelpers)

lib/api/notes.rb

@@ -7,9 +7,7 @@ module API
 
     before { authenticate! }
 
-    NOTEABLE_TYPES = [Issue, MergeRequest, Snippet, Epic].freeze
-
-    NOTEABLE_TYPES.each do |noteable_type|
+    Helpers::NotesHelpers.noteable_types.each do |noteable_type|
       parent_type = noteable_type.parent_class.to_s.underscore
       noteables_str = noteable_type.to_s.underscore.pluralize