This allows admins to:

* Preview emails in the dashboard
* Send preview emails
* Save emails

Fixes #359

Trying to access the dashboard index page directly used to
redirect the user that are not logged in to the '/admin/'
login page instead of the Oscar login.
I implemented a new decorator ``staff_member_required`` that is
based on the one provided in ``django.contrib.admin`` and the
``redirect_to_login`` decorator in ``dango.contrib.auth``. It
redirects the user to the '/accounts/login/' page an uses the
``next`` GET parameter to redirect the user back to the
dashboard index page after successful redirect.

Features:
* Basic crud
* Stats view

Use the fns in app.dashboard.nav to edit