This patch updates the `NotificationMessage` element in `notificationbox.js` so that it extends our newer `moz-message-bar` component instead of the deprecated `message-bar` component. Many of the changes are just dealing with the implications of making things async (so that we can ensure `moz-message-bar.mjs` gets imported). I tried to break out places where I modified related code and tests into separate patches to mitigate some of the review pain here.
This patch solves a longstanding issue where we were loading `in-content/common-shared.css` in the chrome since it gets used by the `message-bar` element. It also makes some small visual changes to our infobars (slight outline, icon colors, adds a bit of spacing).
Differential Revision: https://phabricator.services.mozilla.com/D189872
614f900ec6