Fix first-time contributor notes not rendering

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31117 enabled the
HashInefficientHash Rubocop rule that was fooled by the special
implementation of `SpecialRole`. We fix this by introducing a
`value?` method and adding unit tests.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65383

app/models/note.rb

@@ -27,6 +27,10 @@ class Note < ApplicationRecord
       def values
         constants.map {|const| self.const_get(const)}
       end
+
+      def value?(val)
+        values.include?(val)
+      end
     end
   end
 

changelogs/unreleased/sh-fix-special-role-error-500.yml

+---
+title: Fix first-time contributor notes not rendering
+merge_request: 31340
+author:
+type: fixed

spec/models/note_spec.rb

@@ -913,6 +913,22 @@ describe Note do
     end
   end
 
+  describe '#special_role=' do
+    let(:role) { Note::SpecialRole::FIRST_TIME_CONTRIBUTOR }
+
+    it 'assigns role' do
+      subject.special_role = role
+
+      expect(subject.special_role).to eq(role)
+    end
+
+    it 'does not assign unknown role' do
+      expect { subject.special_role = :bogus }.to raise_error(/Role is undefined/)
+
+      expect(subject.special_role).to be_nil
+    end
+  end
+
   describe '#parent' do
     it 'returns project for project notes' do
       project = create(:project)