Change react-helmet to react-helmet-async

[jthcast]

react-helmet has causing below console error:

Warning: Using UNSAFE_componentWillMount in strict mode is not recommended and may indicate bugs in your code. See https://fb.me/react-unsafe-component-lifecycles for details.

* Move code with side effects to componentDidMount, and set initial state in the constructor.

Please update the following components: SideEffect(NullComponent)

Please check this issue nfl/react-helmet#548.
And react-helmet has async issue too. of course gatsby is not SSR framework but has problem potential.
react-helmet-async is solved this problem and gatsby has gatsby-plugin-react-helmet-asnyc.

what about change react-helmet to react-helmet-async?

[gatsbot]

Hey, @jthcast

Thank you for your pull request!

This repo is now a read-only repo that's synced from the main Gatsby monorepo at https://github.com/gatsbyjs/gatsby/.

We've moved all our starters to https://github.com/gatsbyjs/gatsby/tree/master/starters so changes to starters are made there.

Please checkout our contribution docs & recreate your PR against the starter directory in monorepo.

https://www.gatsbyjs.org/contributing/how-to-open-a-pull-request/

Thanks again!

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