closes https://github.com/TryGhost/Team/issues/1479
- updated post adapter to append `?newsletter_id=xyz` when passed a `newsletterId` adapterOption
- updated editor save task to pass `options.newsletterId` through as `adapterOptions.newsletterId`
- set up `post.newsletter` relationship ready for handling embedded newsletter association from the API
- explicitly deleted when serializing back to the API as it doesn't yet ignore the attribute
- updated `<GhPublishmenu>` for newsletter support
- fetches newsletters on first render so they are available in the dropdown
- sets "default" (first in the ordered list) newsletter as the initially selected newsletter
- adds newsletter dropdown to draft publish menu
- passes `newsletterId` option to editor save task when it's set
This is a minimal implementation for testing. Not included:
- correct free/paid member counts based on selected newsletter
- correct member count in confirmation modal
- indication of selected newsletter for scheduled post
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