Add StateNote synthetic note

app/models/state_note.rb

+# frozen_string_literal: true
+
+class StateNote < SyntheticNote
+  def self.from_event(event, resource: nil, resource_parent: nil)
+    attrs = note_attributes(event.state, event, resource, resource_parent)
+
+    StateNote.new(attrs)
+  end
+
+  def note_html
+    @note_html ||= "<p dir=\"auto\">#{note_text(html: true)}</p>"
+  end
+
+  private
+
+  def note_text(html: false)
+    event.state
+  end
+end

spec/models/state_note_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe StateNote do
+  describe '.from_event' do
+    let_it_be(:author) { create(:user) }
+    let_it_be(:project) { create(:project, :repository) }
+    let_it_be(:noteable) { create(:issue, author: author, project: project) }
+
+    ResourceStateEvent.states.each do |state, _value|
+      context "with event state #{state}" do
+        let_it_be(:event) { create(:resource_state_event, issue: noteable, state: state, created_at: '2020-02-05') }
+
+        subject { described_class.from_event(event, resource: noteable, resource_parent: project) }
+
+        it_behaves_like 'a system note', exclude_project: true do
+          let(:action) { state.to_s }
+        end
+
+        it 'contains the expected values' do
+          expect(subject.author).to eq(author)
+          expect(subject.created_at).to eq(event.created_at)
+          expect(subject.note_html).to eq("<p dir=\"auto\">#{state}</p>")
+        end
+      end
+    end
+  end
+end