Gatsby: Release beta version of v2

[m-allanson]

• [x] Create next.gatsbyjs.org and point it to v2
• [x] Do a new v2 alpha release
• [x] Do a new v1 release
• [x] Merge master into v2
• [x] Test v2 branch
• [x] Branch v1 from master
• [x] Add note about v2 beta to top of readme
• [x] Add link to migration guide to top of readme
• [x] Update v2--- netlify links to next.gatsbyjs.org
• [x] Force pull master into v1
• [x] Change netlify to build gatsbyjs.org from v1 branch
• [x] add a robots.txt to the v2 branch (#5952)
• [x] Add a banner pointing from next.gatsbyjs.org to gatsbyjs.org (#5915)
• [x] Add docs on which branches to use for PRs while v2 is in beta (#5933)
• [x] Resolve #5899
• [x] Do a fresh master -> v2 merge
• [x] Publish final alpha release
• [x] Merge v2 into master
• [x] Update next.gatsbyjs.org to point to master
• [x] Switch to lerna v3
• [x] Publish beta 1 to npm
• [x] Merge v2 beta blog post to v1 branch

[m-allanson]

Set up new.gatsbyjs.org as a generic "upcoming version of Gatsby" preview site? See new.babeljs.io.

[nihgwu]

what about next.gatsbyjs.org as we are using next tag for the coming v2

[KyleAMathews]

👍 to next since that's reusable. Also Babel use(d|s) new because it's a site rebuild and not specifically for the next version of Babel.

[KyleAMathews]

Beta is out!!! Thanks @m-allanson and @pieh and many others for all your contributions! Let's push on to a v2.