title: Sorl-thumbnail not generating thumbnails in '/cache' folder
body: <p>I am trying to use sorl-thumbnail in my templates but I have been unable to successfully generate the thumbnails in the proper '/cache' directory that 'sorl-thumbnail' uses to retrieve thumbnails.</p>
  
  <p>It seems that this problem has happened to a number of people before, however previous solutions have not worked for me.  I have run <code>./manage.py migrate</code> and I have ensured that I am using the sorl.thumbnail.ImageField rather than the default django.db.models.ImageField in my models.py and I have tried using <code>./manage.py thumbnail cleanup</code> and <code>./manage.py thumbnail clear</code>.  I have also read that memcached could be interfering with things, but even with all mentions of memcached commented out, I can't get sorl-thumbnails to work.</p>
  
  <p>For reference, I am running my project on an Ubuntu 18.04.2 apache2 server hosted by DigitalOcean.</p>
  
  <p>html:</p>
  
  <pre><code>{% for obj in object_list %}
      ...
      {% thumbnail obj.image "75x75" as im %}
          &lt;img class = "artwork" src="{{im.url }}"/&gt;
      {% endthumbnail %}
      ...
  {% endfor %}
  </code></pre>
  
  <p>settings.py:</p>
  
  <pre><code>INSTALLED_APPS = [
      'django.contrib.admin',
      'django.contrib.auth',
      'django.contrib.contenttypes',
      'django.contrib.sessions',
      'django.contrib.messages',
      'django.contrib.staticfiles',
      'sorl.thumbnail',
      'posts',
      'login',
  ]
  
  ...
  
  STATIC_URL = '/static/'
  
  STATICFILES_DIRS = [
      os.path.join(BASE_DIR, "/home/arthouse/workspace/code/side_proj/assets/")
  ]
  
  STATIC_ROOT = os.path.join(os.path.dirname(BASE_DIR), "/home/arthouse/workspace/code/side_proj/static_cdn/")
  
  MEDIA_URL = "/media/"
  MEDIA_ROOT = os.path.join(os.path.dirname(BASE_DIR), "/media_cdn")
  </code></pre>
  
  <p>models.py:</p>
  
  <pre><code>from sorl.thumbnail import ImageField
  
  class Post(models.Model):
      image = ImageField(upload_to=upload_location,
          null=True,
          blank=True,
          width_field="width_field",
          height_field="height_field")
  </code></pre>
  
  <p>I have set <code>THUMBNAIL_DEBUG=True</code> but no output comes from this.  I can see that thumbnails are generated, (eg: media/cache/6d/13/6d13206b5207bf2b6234295b749f1419.jpg) but there is never a /cache directory created within my MEDIA_ROOT which leads to a 404 error.</p>
  
  <p>I'm happy to share more code snippets if that would help!</p>