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.
Merge pull request #643 from codeforamerica/590-popup-types

Fixes #590 - refactors popups to have types