Support for custom post types in post status events.

### Context
The `When post changes status` event has a dropdown to select post types to apply for this, it is expected that custom post types would be included in the dropdown, but is not, because the event and UI is registered a bit earlier than convenient.

### Steps to reproduce
- Create a new workflow
- Choose `When post changes status`
- Try selecting a custom post type from the list of post types

Actual: You can only see the built-in post types.
Expected: You can see all registered custom post types.