There are no changes to appearance of the 'Preview this alert' button or
what it does, but this stops a CSRF token appearing in the query
string when you click the button. We don't need a CSRF token - it's
a simple GET request which doesn't change any data. Before, we had a form
with `method="get"` but because we were using a `page_footer` a CSRF
token was being added.
We can replace both the `<form>` element and `page_footer` with a
`govukButton`. This means that we make a GET request with no CSRF token
without changing the appearance of the button.
22d25b7a1c