Fixes #590 - refactors popups to have types

Fixes #590 - refactors popups to have types. To decouple the popups, a
Default and Feedback type were extracted from the popup-manager. The
popup manager is slimmed down accordingly.
Additionally:
- Makes feedback form use existing global alert
infrastructure instead of defining its own, which removes a lot of
essentially redundant styles related to the alerts.
- Adds optimizations to popups so certain values that don't change aren't looked up each time the popups are rendered.
- Fixes issue where popups could become misaligned on the page relative to their arrows when resizing the window.
- Creates a new layer for top-most alerts.