(fix) removes duplicate ARIA landmark
(fix) removes duplicate ARIA landmark

https://trello.com/c/NCh9Ld6a/11-duplicate-unlabelled-landmarks

Removes one of two `role=“banner”` attributes from the page, leaving in place the one that best matches the [description of the landmark’s use](https://a11yproject.com/posts/aria-landmark-roles/).

Adds an `aria-label` to the header that contains the name of the blog.

dxw/govuk-blogs,dxw/govuk-blogs,dxw/govuk-blogs