Add system_note_icon mapping for epic actions

Related to #4262

app/helpers/system_note_helper.rb

 module SystemNoteHelper
+  prepend EE::SystemNoteHelper
+
   ICON_NAMES_BY_ACTION = {
     'commit' => 'commit',
     'description' => 'pencil',

ee/app/helpers/ee/system_note_helper.rb

+module EE
+  module SystemNoteHelper
+    extend ::Gitlab::Utils::Override
+
+    EE_ICON_NAMES_BY_ACTION = {
+      'epic_issue_added' => 'issues',
+      'epic_issue_removed' => 'issues',
+      'epic_issue_moved' => 'issues',
+      'issue_added_to_epic' => 'epic',
+      'issue_removed_from_epic' => 'epic',
+      'issue_changed_epic' => 'epic'
+    }.freeze
+
+    override :system_note_icon_name
+    def system_note_icon_name(note)
+      EE_ICON_NAMES_BY_ACTION[note.system_note_metadata&.action] || super
+    end
+  end
+end

ee/changelogs/unreleased/ee-jprovazn-epic-icon.yml

+---
+title: Add icons to epic system notes issue actions
+merge_request:
+author:
+type: fixed

ee/spec/serializers/epic_note_entity_spec.rb

@@ -16,4 +16,13 @@ describe EpicNoteEntity do
   it 'exposes epic-specific elements' do
     expect(subject).to include(:toggle_award_path, :path)
   end
+
+  context 'on a system note' do
+    let(:note) { create(:system_note, noteable: epic, author: user) }
+    let!(:note_metadata) { create(:system_note_metadata, note: note, action: 'epic_issue_added') }
+
+    it 'sets system_note_icon_name for epic system notes' do
+      expect(subject[:system_note_icon_name]).to eq('issues')
+    end
+  end
 end