186: Previews iframe refactor

[rpeterman-gp]

Closes #186

• Removes logic to create preview authentication cookie.
• Removes Wp GraphQL JWT Authorization plugin.
• Adds TW Previews plugin that:
  • Creates a custom preview route.
  • Creates a preview route template that loads front-end preview routes in an iframe.
  • Communicates with the front-end preview pages with post message events to receive the query needed to fetch data, then send the fetched data to the front-end to render.
  • Preview route polls query for changes and sends updated data to front-end (auto updates while editing)
  • Preview route requires authenticated user.

To Review

• [x] Checkout Branch.
• [x] Run npm run local (or npm run refresh if you want a fresh database.)
• [x] Log into http://the-world-wp.lndo.site/wp-login.php.
• [x] Start Review of PRX/theworld.org#460

...then...

• [x] Create or edit any story.
• [x] Click the "Preview in new tab" link. (You can keep that tab in the current window or drag it to its own window.)
• [x] Ensure preview window loads with current state of story.
• [x] Makes some changes to story.
• [x] Watch for an autosave to happen, then check that the preview is updated with your changes.
  • Autosaves may not occur. Click "Save Draft" button to immediately push changes.
  • Click "Preview in new tab" link when any of the following quirks occur:
  • ACF fields values may revert to older revisions in the preview. ie. feature stories may switch to preview as default.
  • Taxonomy changes will not show up in the preview until a core content field is changed.
• [x] Repeat for Episode preview.
• [x] Repeat for Segment preview.
• [x] Repeat for Page preview. (Pages do not have a "Save Draft" button, so auto updates only sync when a autosave occurs.)
• [x] Copy URL from any preview window and past it into a new incognito window or another browser you are not logged into WP with.
• [x] Ensure you get a 403 not authorized response.