feat: serve page with no header nor footer

[wesleyboar]

Overview

Allow serving a page with no header and no footer.

Changes

• adds template content.html
• adds blocks header and footer
  ^{so they can be emptied by the new template}

Testing

0. In settings_custom.py or settingslocal.py, add:

   CMS_TEMPLATES = (
       ...
       ('content.html', 'Content Only'),
   )

1. Open any page that has content.
2. Append ?template=content.html&toolbar_off to the URL.
3. Verify page header and page footer are not shown.

UI

normal URL	?template=content.html