refs https://github.com/TryGhost/Team/issues/1528
- adds "opt-in existing" toggle to newsletter modal that's only shown when creating a newsletter
- defaults to true
- updated newsletter save flow to show confirmation before creation
- alters message to reflect auto-subscribe selection
- count of existing subscribed members is not implemented as it's not yet supported by the API
- updated newsletter adapter and save flow to use auto opt-in selection
- when option is checked the save URL is changed to `POST /members/?opt_in_existing=true`
- modified task button component to ignore a task return value of `canceled` so when it's received the buttons returns to the idle state instead of showing a saved or failed state
- used by save routine when the "Back to edit" button is clicked in the create confirmation modal
