Spinner is persistent even if user cancels the new post action

[bacoords]

Because "add new post" triggers a link in the same window, if you have unsaved changes in your post, the browser will pop-up a confirmation before you leave. If you cancel and choose not to go to the new post screen, you come back to the editor, but the spinner is still spinning.

Possible options:

• Open new post in new window? (confirm about A11y)
• Check block editor data store for unsaved changes and disable button?
• Remove the spinner for the basic 'add new post' button, since it might not really need it.