Switch from REST to GraphQL for Gatsby to Telescope

[humphd]

What would you like to be added:

Let's use the gatsby-source-graphql plugin to allow using our Telescope GraphQL API instead of getting posts/feeds/etc from the REST API.

Why would you like this to be added:

Gatsby uses GraphQL all the way through, so we might as well embrace that. Also, we have the GraphQL endpoint on Telescope, so we might as well embrace that too!