[7.2.x] Notifications Slide-Out Does Not Use History API

The new notifications slide-out should leverage the History API so that pressing the back button will close the notifications rather than navigating back a page. This is especially true on mobile, where the notifications slide-out covers the entire screen, appearing to be a new page for all intents and purposes.

https://developer.mozilla.org/en-US/docs/Web/API/History/pushState