Make a full width base template

We need this because the top section of the new product page will be
going full-bleed.

app/templates/fullwidth_template.html

@@ -0,0 +1,7 @@
+{% extends "admin_template.html" %}
+
+{% block fullwidth_content %}
+  <main role="main">
+    {% block maincolumn_content %}{% endblock %}
+  </main>
+{% endblock %}