Swap notification close link for a button

No issue

- Swap out notification <a> close link for <button> (we gain a native hover state)
- Remove element selector from tests, to is only looks for the class now

No style changes are needed, but https://github.com/TryGhost/Ghost-UI/commit/087868c1c99b84e0c5b9701202418ce7b477b18c was done in tandem