Downloads warning styling improvements (use moz-message-bar)

In #41718 (closed) and #41800 (closed) we added a download warning to about:downloads and places.xhtml.

In the merge request we (cc @donuts @ma1) weren't quite happy with the styling:

1. The Library button has different old styling. This is just because the default button styling is not in scope. I'm not sure it is worth importing all those button styling rules.
2. The icon is center aligned with the message box, rather than aligned with the bold title. The mozilla message-bar element doesn't have an easy way to control this because the icon lives in a shadow root, so doesn't share the styling rules with the rest of the content.

In the first case, I think we can probably leave this until mozilla imports the common.css styling into places.xhtml, or just re-implements that since it is pretty old. The (possibly-inactive) bugzilla 1788947 may be relevant.

In the latter case, it is do-able by us if we modify the <message-bar> widget code, but mozilla might also be interested in doing this if they want to start using the widget in a similar way. E.g. in about:preferences there are some multi-line message boxes where they might want to eventually use this.