Fixes #661 - closes feedback popup after comment

Fixes #661 - closes the feedback popup after submitting a comment in
the feedback form. To accomplish this the feedback-form-manager was
refactored to use the EventObserver mixin and also to have a type that
could be instantiated. Unnecessary ids related to the feedback form
were also removed.
Merge pull request #681 from codeforamerica/661-close-popup-after-send

Fixes #661 - closes feedback popup after comment