i18n [nfc]: Standardize more todo-i18n comments, from free-form

Author: gnprice

lib/widgets/message_list.dart

@@ -1410,5 +1410,5 @@ class MessageWithPossibleSender extends StatelessWidget {
   }
 }
 
-// TODO web seems to ignore locale in formatting time, but we could do better
+// TODO(i18n): web seems to ignore locale in formatting time, but we could do better
 final _kMessageTimestampFormat = DateFormat('h:mm aa', 'en_US');

lib/widgets/profile.dart

@@ -200,7 +200,7 @@ class _ProfileDataTable extends StatelessWidget {
         // TODO(server): The value's format is undocumented, but empirically
         //   it's a date in ISO format, like 2000-01-01.
         // That's readable as is, but:
-        // TODO format this date using user's locale.
+        // TODO(i18n) format this date using user's locale.
         return _TextWidget(text: value);
 
       case CustomProfileFieldType.shortText: