Bug 1067228 - [Messages][Refactoring] Move subject management to a separate component. r=julien

Ok, since we don't have agreement on that will keep it static (except for placeholder, I think it still better to remove it + it has data-l10n-id), but still will try to avoid having class on form (if possible :))

JFR, the main reason why I wanted it to be dynamic is that I believe subject isn't frequently used to be in markup by default and yes I wanted to use webcomponents eventually and lazy load it including markup.