Introduce private docs and flatpages preview (#26)

* Add support for private documents
* Fix migrations after merge
* Rebase fixtures
* Filter private decoder docs
* Enable preview of unpublished pages

ram/portal/templates/cards/rolling_stock.html

@@ -1,7 +1,7 @@
 {% load static %}
         <div class="col">
           <div class="card shadow-sm">
-            {% if d.item.image.count > 0 %}
+            {% if d.item.image.exists %}
             <a href="{{d.item.get_absolute_url}}"><img class="card-img-top" src="{{ d.item.image.first.image.url }}" alt="{{ d }}"></a>
             {% else %}
             <!-- Do not show the "Coming soon" image when running in a single card column mode (e.g. on mobile) -->