Handle started milestone persisted on board

ee/app/models/ee/board.rb

@@ -34,8 +34,11 @@ module EE
     def milestone
       return nil unless parent.feature_available?(:scoped_issue_board)
 
-      if milestone_id == ::Milestone::Upcoming.id
+      case milestone_id
+      when ::Milestone::Upcoming.id
         ::Milestone::Upcoming
+      when ::Milestone::Started.id
+        ::Milestone::Started
       else
         super
       end

spec/ee/spec/models/ee/board_spec.rb

@@ -33,6 +33,12 @@ describe Board do
         expect(board.milestone).to eq Milestone::Upcoming
       end
 
+      it 'returns Milestone::Started for started milestone id' do
+        board.milestone_id = Milestone::Started.id
+
+        expect(board.milestone).to eq Milestone::Started
+      end
+
       it 'returns milestone for valid milestone id' do
         milestone = create(:milestone)
         board.milestone_id = milestone.id