wip: new features for Ghost source plugin

[nocash]

Rewrites a large portion of the plugin in order to properly link related values to their GraphQL node objects.

Todo

• Remove the GhostPage node type (Note: the GhostPage node type has been readded after the updates for the v2 API).
• Use createRemoteFileNode to copy images to local file system
• Add Babel and target Node 4
• Add Prettier config to maintain code style consistency
• Replace the large "does roughly the right thing" test with smaller, focused tests
• Update README

Significant changes

• Replaced certain object properties with links to the related node(s). post.tags, post.primary_tag, post.authors, and post.primary_author now return the actual nodes rather than containing a limited copy of the data.

• Removes the GhostPage node type (explanation below). (Note: the GhostPage node type has been readded after the updates for the v2 API).

• Added a posts collection to GhostTag and GhostAuthor nodes that contain node references to the posts they are assigned to (e.g. author.posts returns all posts written by that user).

• Adds a GhostMedia node type that is used for post.feature_image, tag.feature_image, user.profile_image, and user.cover_image. The GhostMedia type contains a src field the retains the original URL from those fields but additionally downloads the image and creates a localFile node that can be used with the gatsby-image package.

• Queries the /tags and /users endpoints of the Ghost API to get a full set of data rather than inferring it from the posts collection. Previously, tags that were not added to any posts were never passed to Gatsby, and the same was true for users.

Removing the GhostPage node type

Note: The GhostPage node type was removed when working with the v0.1 API but has been readded after updating to the v2 API to maintain parity with the API resources (see https://docs.ghost.org/api/content/#resources). The information below is being kept for posterity but may be removed later.

GhostPage is not significantly different from GhostPost— the only difference being that post.page is true instead of false— and now that nodes and backreferences are working correctly it complicates what should be trivial queries by requiring inline fragments in order to select fields from otherwise identical objects. E.g.:

query {
  ghostTag(id: {eq: "..."}) {
    name
    posts {
      ... on GhostPost {
        title
      }
      ... on GhostPage {
        title
      }
    }
  }
}

We can remove the GhostPage type from the source plugin and we leave it to the consumers to decide how they want to handle pages. This is easily done in gatsby-node.js:

exports.createPages = ({ graphql, actions }) => {
  const { createPage } = actions
  return new Promise((resolve, reject) => {
    graphql(`
      {
        allGhostPost {
          edges {
            node {
              slug
              page
            }
          }
        }
    `).then(result => {
      result.data.allGhostPost.edges
        .filter(({node}) => !node.page)
        .forEach(({ node }) => {
          createPage({
            path: `post/${node.slug}`,
            component: path.resolve('./src/templates/post.tsx'),
          })
      })

      result.data.allGhostPost.edges
        .filter(({node}) => node.page)
        .forEach(({ node }) => {
          createPage({
            path: `page/${node.slug}`,
            component: path.resolve('./src/templates/page.tsx'),
          })
      })

      // ...
    })
  })
}

[nocash]

The changes included have outgrown the scope of #1 so I'm closing it in favor of this PR.