update react tutorial to support apollo client v3

[praveenweb]

The current react tutorial uses Apollo Client v2 stable. v3 changes some of the APIs. We can update the tutorial with the necessary changes.

• cache.read and cache.write are replaced with cache.modify.

// TODO: Add more changes here.

[kriswep]

I could do this.
Currently, there are two react tutorials here, react-apollo and react-apollo-hooks.
The version on https://hasura.io/learn/ seems to be the `react-apollo-hooks, correct?

[praveenweb]

@kriswep - Yes! react-apollo-hooks is the latest one and maintained. The react-apollo is the old one that uses Apollo Components and we don't maintain that anymore and is there just for reference code.

Apollo has released v3.0.0-rc.0. Might be good to break down what changes for the current tutorial.

High level, I see that

• Packages have changed (@apollo/client replaces apollo-boost and a few others).
• Managing cache needs to be updated.
• Optimistic UI usage might need an update.
• Verify if there are changes to subscriptions.

[kriswep]

Yes, nice.
I‘d follow apollos‘ migration guide and figure things out from there.