flake8
passed.
JSHint
passed.
Convert header to react.
Review Request #9558 — Created Jan. 31, 2018 and submitted
This change converts the header to use react internally. It's still
wrapped by a Backbone view, but that will change in the next commit,
which moves the main application frame to use react.
Verified that the header functioned correctly, including the toggle for
the "manage" state.
