update createdAt/updatedAt state after save

src/client/js/services/PageContainer.js

@@ -243,6 +243,8 @@ export default class PageContainer extends Container {
       revisionIdHackmdSynced: page.revisionHackmdSynced,
       hasDraftOnHackmd: page.hasDraftOnHackmd,
       markdown: page.revision.body,
+      createdAt: page.createdAt,
+      updatedAt: page.updatedAt,
     };
     if (tags != null) {
       newState.tags = tags;