refs https://github.com/TryGhost/Team/issues/1121
Double-clicking the delete button in the confirmation modal could trigger a second attempt to delete the post/page when it had already been deleted resulting in an ugly/incomprehensible error bubbling up.
- added `{drop:true}` to the task instance so it can't be called again whilst it's currently running
- added a guard for `isDeleted` so we don't attempt to destroy an already deleted record
- added a `true` return so the button indicates a successful state rather than an error state whilst the modal is closing
