get rid of mb2's .forget() invocations

[ctm]

Get rid of all the calls to .forget() that mb2's web client uses.

I didn't think that calling .forget on a Timeout leaks memory, but it does, at least until we can invoke wasm-bindgen with --weak-refs. There's currently no way to tell web-pack to invoke wasm-bindgen that way though.

I believe we're currently only calling .forget() for resize handlers (i.e. one per window, so that leakage doesn't really matter) and Timeouts. It's the Timeouts that could be an issue, but it's trivial to fix for the resize handlers, since there's an obvious place to store the closure. Each of the Timeouts will require a bespoke solution.

[ctm]

Done. Deploying now.