Assistive technologies use the `<main>` element to navigate around a document.
In `<main>` their users expect to find:
> [content] unique to the document, excluding any content that is
> repeated across a set of documents such as sidebars, navigation links,
> copyright information, site logos, and search forms…
— https://developer.mozilla.org/en/docs/Web/HTML/Element/main
Previously, the `<main>` element also wrapped the sidebar navigation. This
commit moves the `<main>` element to only wrap the content of the page when the
page has a navigation sidebar.
This commit also removes the `page-container` class which wasn’t being used for
anything.
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.
