Increase Varnish client session timeout from 5 (default) to 15 seconds

We received a report of 503 errors being returned on sign-in attempts. It seems that there's a client session timeout option in Varnish that defaults to 5 seconds, and so it's possible that this timeout might be getting reached by flaky connections to cause this error. This bumps the default timeout to 15 seconds in order to be a bit more lenient, which should drop the frequency of these timeout errors.