turns out that we're only using errorBanner with a static message, and
it's also full of rich html content. This means that it's probably
better to put it in the html templates with other content, rather than
hidden away in js files if we can help it.
Since there are two places, had to dupe the error message but i think
that's fine as i don't anticipate this error message being used in
significantly more places.
making it a string is a bit gross and means we don't get nice syntax
highlighting on it, but as it needs to be passed in to a jinja macro
that's the way it has to go unfortunately.
this ensures it's reusable by other components, and easier to unit test
by isolating the separate concerns
note: this is not in Modules since that's designed for classes that are
then bound to an element in the DOM as indicated by a data-module
attribute. This will just live at the window.GOVUK level since we want
there to only ever be one `.banner-dangerous` warning.
