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.

lib/api/discussions.rb

@@ -7,9 +7,7 @@ module API
     before { authenticate! }
-    NOTEABLE_TYPES = [Issue, Snippet, 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

lib/api/helpers/notes_helpers.rb

@@ -3,6 +3,12 @@
 module API
   module Helpers
     module NotesHelpers
+      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])

lib/api/notes.rb

@@ -7,9 +7,7 @@ module API
     before { authenticate! }
-    NOTEABLE_TYPES = [Issue, MergeRequest, Snippet].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