[feat]: add Gatsby Cloud functionality and "hot reloading"

[DSchau]

Summary

Right now -- this plugin has several areas that could be augmented to better support the Gatsby ecosystem, particularly with Gatsby Cloud. Specifically:

1. Images are sourced from static.ghost.org rather than tying into createRemoteFileNode functionality (See #5)
2. No ability to "live update" data from Ghost, e.g. when content is created, updated or deleteed

Let's focus on #2 for this issue.

Live Updating

Gatsby Cloud uses webhooks to synchronize content from the CMS with the running Gatsby application. Ghost already supports webhooks, so we're part of the way there towards supporting hot reloading of data.

How it works under the hood is that the sourceNodes function in gatsby-node.js is re-invoked whenever we receive a web hook. What's missing now is some way to incrementally know what has been changed, and how to take the current state of the GraphQL schema and update it to match the new Ghost data.

To best support plugin authors, we've put together an Integration Guide that outlines several approaches in updating a plugin to have great Gatsby Cloud support. The eventual result of this work would be:

1. This plugin has a way to request changed data
2. This plugin (when sourceNodes is invoked) calls createNode, deleteNode, and touchNode to get the schema consistent with Ghost

@JohnONolan question -- does the webhook that's triggered receive a payload? That could be a relatively low-effort way to get the updated content!

Adding Gatsby Cloud as an Integration

For anyone else who may be working on this, adding Gatsby Cloud will look something like this:

[DSchau]

For anyone who works on this, we're happy to give extended trials of Gatsby Preview to best support the necessary work to get this plugin supported. We'd love to see Ghost as a first-class integration!

[Kerumen]

Hello everyone,

I've just setup a blog with Ghost and Gatsby and was wondering how to "reload" data from Ghost when deployed in production. As Gatsby is a static site, it must be rebuild each time we update data on Ghost?

This is a huge pain point for blog websites as it's updated very frequently.

Thanks for your answer :)

Edit: OK, I found the official docs for Netlify to trigger rebuilds any time content is updated using webhooks.