Reduce usage of linkPrefix in 1.x

[KyleAMathews]

A common problem with building sites is that you may deploy the same site in dev/testing/prod with different URL roots. Often it's just / but perhaps you're deploying to Github pages at YOUR_NAME.github.io/PROJECT_NAME in which case you'd need to prefix all links with /PROJECT_NAME.

In 0.x we just run everything through a helper function linkPrefix in the included module gatsby-helpers. This works but it's a bit annoying to have to surround every link on your site with this special function. Especially when Webpack + React Router can do the heavy lifting for us with publicPath in Webpack and basename in React Router.

I've already fixed this in Webpack in the 1.0 branch but adding basename support to react-router still needs to happen.

[KyleAMathews]

Fixed now in v1. No more need to use prefixLink!