Use <main> element in semantically appropriate way

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.

app/templates/withoutnav_template.html

@@ -1,6 +1,8 @@
 {% extends "admin_template.html" %}
 
 {% block fullwidth_content %}
-  {% include 'flash_messages.html' %}
-  {% block maincolumn_content %}{% endblock %}
+  <main role="main">
+    {% include 'flash_messages.html' %}
+    {% block maincolumn_content %}{% endblock %}
+  </main>
 {% endblock %}