129: Create `tw-newsletters` plugin

[rpeterman-gp]

Closes #129

• create newsletter post type
• add acf fields for newsletters (configs in theme directory)
• clean up redundant props in custom post types (code review)

To Review

• [x] Checkout Branch.
• [x] Run npm run local (or npm run refresh if you want a fresh database.)
• [x] Sign in as an admin to http://the-world-wp.lndo.site/wp-admin.

...then...

• [x] Ensure you see Newsletters link in menu.
• [x] Go to Newsletters then click Add New button.
• [x] Reference Drupal admin to get values needed to recreate the "Top of The World" newsletter sign up. (Feature image can be added from media library, search for marco-meeting.)
• [x] Note default values for Opt-in Text and Button Label fields should be adequate in most cases.
• [x] Ensure field descriptions are helpful.
• [x] Click Publish and confirm.
• [x] Go to GraphQL IDE and enter this query:

  
  query getNewsletters {
  newsletters(first: 10) {
  nodes {
    id
    link
    title
    content
    excerpt
    featuredImage {
      node {
        ...ImageProps
      }
    }
    newsletterOptions {
      listId
      buttonLabel
      optInText
    }
  }
  }
  }

fragment ImageProps on MediaItem { id sourceUrl mediaItemUrl altText mediaDetails { width height } }

- [x] Submit query and ensure the data includes the newsletter you just created.