Did most of this work in:
https://github.com/alphagov/notifications-admin/pull/1118
> In pages specific to a service (e.g. dashboard and sub pages) the
> title needs to distinguish which service it applies to. This is mainly
> to give context to screen reader users who could be managing multiple
> services.
>
> Implementing this uses template inheritance:
>
> `page_title` includes `per_page_title` includes `service_page_title`
>
> ‘GOV.UK Notify’ is inserted into every page title.
>
> Pages that set `service_page_title` get the service name inserted too.
Banners should always be the first thing on the page.
Because headers already have padding we don’t want to put padding on the
container.
So banners should also have top padding to distance then from the red bar.
They should also sit in the 3/4 column if the page has side navigation. This
commit adds a new template (`withoutnav_template.html`) which extends
`admin_template.html`. All views then extend one or the other, never the
`admin_template.html` directly. This means that `admin_template.html` doesn’t
have to make decisions about where the flash messages are displayed.
