Once the new moz-message-bar component is more or less complete/ready to use we should start using it in places where we're currently using the older message-bar element. Once message-bar is no longer being used we can remove all the associated code (component file, stories, etc.)

We can track removing message-bar from different places in the codebase via additional bugs filed under this metabug. There are a number of additional places in the app where we can use the moz-message-bar component to replace existing UI elements that look like a message bars, but don't actually use the message-bar component. Those tasks will be tracked separately as they have no bearing on our ability to remove the old message-bar code from the codebase.