Use retry in client form submission

[nukemberg]

It's expected that some users will not be able to get a reply from the server; When that happens, they will retry but clicking the "submit" button. It's better if the client code retries automatically since that allows control of retry rates. If possible, a backoff sleep interval (with random splay) will be used as well to reduce load and break client coordination. (trpc only allows specifying the number of retries)