Add mechanics to add advertising blocks.

https://github.com/madewithbytes/us_ignite/issues/157

Only and advertising block can be shown accross the site.
Add manager helper to return the latest featured ``Advert``.

https://github.com/madewithbytes/us_ignite/issues/157
Make sure there is a single published featured ``Advert``.

https://github.com/madewithbytes/us_ignite/issues/157
Implement template tag to show the latest ``Advert``..

https://github.com/madewithbytes/us_ignite/issues/157

the template tag takes a template parameter to determine
how does the box should be rendered. Usage::

     {% advertising_box "advertising/object_block.html" %}
Add advertising box in the ``blog`` via a temlate tag.