[1.5.x] Fixed #19779 -- Checked contrib.sites presence in RedirectFallbackMiddleware

Thanks Aymeric Augustin for the report and directions for the patch.
Backport of 2ed90eac from master.

docs/ref/contrib/redirects.txt

@@ -13,11 +13,12 @@ Installation
 
 To install the redirects app, follow these steps:
 
-1. Add ``'django.contrib.redirects'`` to your :setting:`INSTALLED_APPS`
-   setting.
-2. Add ``'django.contrib.redirects.middleware.RedirectFallbackMiddleware'``
+1. Ensure that the ``django.contrib.sites`` framework
+   :ref:`is installed <enabling-the-sites-framework>`.
+2. Add ``'django.contrib.redirects'`` to your :setting:`INSTALLED_APPS` setting.
+3. Add ``'django.contrib.redirects.middleware.RedirectFallbackMiddleware'``
    to your :setting:`MIDDLEWARE_CLASSES` setting.
-3. Run the command :djadmin:`manage.py syncdb <syncdb>`.
+4. Run the command :djadmin:`manage.py syncdb <syncdb>`.
 
 How it works
 ============

docs/ref/contrib/sites.txt

@@ -246,6 +246,7 @@ To do this, you can use the sites framework. A simple example::
     >>> 'http://%s%s' % (Site.objects.get_current().domain, obj.get_absolute_url())
     'http://example.com/mymodel/objects/3/'
 
+.. _enabling-the-sites-framework:
 
 Default site and ``syncdb``
 ===========================

docs/releases/1.5.txt

@@ -653,6 +653,12 @@ Miscellaneous
   Attempting to load it with ``{% load adminmedia %}`` will fail. If your
   templates still contain that line you must remove it.
 
+* Because of an implementation oversight, it was possible to use
+  :doc:`django.contrib.redirects </ref/contrib/redirects>` without enabling
+  :doc:`django.contrib.sites </ref/contrib/sites>`. This isn't allowed any
+  longer. If you're using ``django.contrib.redirects``, make sure
+  :setting:``INSTALLED_APPS`` contains ``django.contrib.sites``.
+
 Features deprecated in 1.5
 ==========================