closes https://github.com/TryGhost/Team/issues/1584
refs https://github.com/TryGhost/Team/issues/1605
- added email limit check to PublishOptions setup
- moved email disabled messaging from the email recipients option to the publish type option
- it was confusing to have the email publish type options disabled without any indication of why, with the message hidden within the closed email recipients option
refs https://github.com/TryGhost/Team/issues/1542
- adds a `PublishOptions` class
- an instance of this class provides everything the UI needs to display and set the publish options relevant to the current post object and overall system state
- the `publish-flow` modal is passed a `PublishOptions` instance when it opens
- as part of the constructor it triggers a background load of any additional data it requires to control available options such as member counts, email limits, and newsletters
- adds a `{{publish-options}}` resource
- sets up and returns `PublishOptions` instance
- passes through service dependencies which are not available directly in the `PublishOptions` class as it's a custom native class outside of Ember's DI management
- used to ensure we can get a clean `PublishOptions` instance any time the passed in `post` object is replaced meaning we don't have to rely on observers and manual teardown/setup
- updated `<PublishManagement>` component
- sets up `publishOptions` property using `@use` and the `{{publish-options}}` resource so reactivity for changing post objects is handled automatically
- uses the `publishOptions.isLoading` property to disable the publish flow trigger button until all of the data required to manage the flow is available
- updated `publish-flow` modal to use some of the initially available `publishOptions` data
