Implement new popovers

No issue

Implements a new popover component (not to be confused with what are now called dropdowns [menus])

Todo:

- [x] Emberify
- [x] Get checked over by people with Über Ember skilz

There's nowhere that needs a popover right now, so this is just to preview and show its markup.
Once the rest of the code is all good, I'll remove this & remove the [WIP] label.