Subreddit shortcuts do not save changes if pre v4.1.5 format

If RESStorage RESmodules.subredditManagers.subredditShortcut.username shortcuts don't have .addedDate properties on all the shortcuts (RES v4.1.2 format, presumably), then RES v4.3.x loads them in with .addedDate = Date[invalid date].  Then, when you go to save them, JSON.stringify breaks on the invalid dates (in Safari, at least - Firefox might be more tolerant) and the user can't save any changes to their subreddit shortcuts.