Update react-helmet syntax to new simplified API

[markmichon]

Closes #1906. Converts most examples, as well as the gatsbyjs.org site source to use the new simplified api in react-helmet. One notable exception being the examples/using-javascript-transforms files, as the example fails to run and probably deserves it's own PR with some updates.

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[jbolda]

Do you have an error output for examples/using-javascript-transforms?

[markmichon]

@jbolda sure thing, it has to do with the query in the index layout.

GraphQL Error Unknown field `allSite` on type `RootQueryType`

  file: /Users/markmichon/_dev/gatsby/examples/using-javascript-transforms/src/layouts/index.js

   1 | 
   2 |   query LayoutBySlug {
>  3 |     allSite {
     |     ^
   4 |       edges {
   5 |         node {
   6 |           siteMetadata {
   7 |             title
   8 |             siteDescr
   9 |             siteAuthor
  10 |             siteEmailUrl
  11 |             siteEmailPretty
  12 |             siteTwitterUrl
  13 |             siteTwitterPretty

The fix is pretty straightforward, changing the above query into:

query LayoutBySlug {
    site {
          siteMetadata {
            title
            siteDescr
            siteAuthor
            siteEmailUrl
            siteEmailPretty
            siteTwitterUrl
            siteTwitterPretty
          }
        }
      }

Then adjusting some of the variable declarations in the file. I can put together a PR specifically for that, with the inclusion of the helmet changes. It looked like a good portion was still WIP so I didn't want to get in the way.

[jbolda]

It's... as finished as it can be pending #1895. This is issue fixed already in that PR. Might have been good to separate it into another PR though. I haven't and didn't plan to update react-helmet as part of that PR though. Whatever you think is best. I just wanted to make sure it wasn't something else I missed.

[markmichon]

Ah, I see. Makes sense. I'll leave it out of this PR and adjust down the road once it makes it's way into master.

[KyleAMathews]

@jbolda allSite was removed recently as there can only be one Site node so the allSite was meaningless. Probably should have waited for a major release for that…

[KyleAMathews]

Nice! Love the new syntax. Thanks for fixing things up!

[jlengstorf]

Hiya @markmichon! 👋

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