feat: upgrade to updated Gatsby plugins + reorganize

[angeloashmore]

Important: This PR should remain a draft until gatsby-source-prismic V4 and gatsby-plugin-prismic-previews V4 is published.

This PR integrates the alpha gatsby-source-prismic V4 and gatsby-plugin-prismic-previews V4 plugins. It also reorganizes the project to be consistent with the updated prismicio/gatsby-getting-started-tutorial repository.

• Update all dependencies
• Add Prettier to automatically format files (npm run format will reformat again)
• Use a leaner ESLint configuration (w/o Prettier)
• Move global CSS imports to gatsby-browser.js and gatsby-ssr.js
• Move Slices to src/slices
• Write Slice fragments within individual Slice files
• Use gatsby-plugin-image for all images

Further rearchitecting can be done with the SliceZone component pending further discussion here: prismicio/prismic-react#86

gatsby-plugin-image image support can be integrated after the following upstream issue is resolved: imgix/gatsby#122

[angeloashmore]

This is ready to review now. The PR contains gatsby-image-plugin and the latest Gatsby + Prismic plugins.

[angeloashmore]

Thanks @faresd, I replied to your comments. Happy to discuss further if needed 👍. Otherwise the PR is ready to merge.