refs https://github.com/TryGhost/Ghost/issues/7515
- add a `retry` icon that better matches button text dimensions
- add a spin animation for the retry icon
- use the new retry icon in the default `gh-task-button` template
- add a "restart animation" method to `gh-task-button` so that repeated clicks on a button appear to register the action when local validation means that there's no transition from failed->running->failed
