Add cleanups to useEffect() calls with callbacks

[divergentdave]

There are a few useEffect() calls that take a promise and call then() on it. (These are in TaskTitle(), AccountName(), and usePromise()). There's also one in ErrorPage, but it's moot because the effect navigates away from the app.) These should return cleanup functions, following the usual cancellation flag pattern. It's possible that rapid clicks, plus HTTP requests completing out of order, could lead to confusing UI interactions, and ignoring promise resolutions from old invocations of effects will fix these potential issues.