feat: move react-helmet to peerDependencies (#2964)
feat: move react-helmet to peerDependencies (#2964)

- Remove react-helmet from dependencies
- Add react-helmet from peerDependencies

BREAKING CHANGE: Requires that react-helmet is installed alongside
gatsby-plugin-react-helmet.

re gatsbyjs/gatsby#2446
ChristopherBiscardi/gatsby,fk/gatsby,fabrictech/gatsby,danielfarrell/gatsby,fabrictech/gatsby,fk/gatsby,Khaledgarbaya/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,0x80/gatsby,mingaldrichgan/gatsby,0x80/gatsby,fabrictech/gatsby,Khaledgarbaya/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,danielfarrell/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,fk/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,danielfarrell/gatsby,Khaledgarbaya/gatsby,0x80/gatsby