Update web client to use futures 0.3 and wasm-bindgen-futures 0.4

[kellpossible]

I think the dependencies for the web client may be a little out of date.

[tomhoule]

There's a WIP PR for this: https://github.com/graphql-rust/graphql-client/pull/327 - it would go further and replace our custom browser HTTP client.

I don't have time to work on this at the moment, but may have in the not too distant future. If someone else wants to do it, PRs are very welcome :)

[kellpossible]

@tomhoule I ended up doing this myself in a copy of the class, I'll use it until the pull request is merged :) https://github.com/kellpossible/coster/blob/6aaa3139bef6dec5d50f0a44ea96f13baa663bf8/gui/src/graphql/client.rs