Add a default card image when no custom one exists (#23)

* Add a default card image when no custom one exists
* Add coming_soon.png source
* Use directly the svg source instead of the png raster

ram/portal/context_processors.py

@@ -0,0 +1,5 @@
+from django.conf import settings
+
+
+def default_card_image(request):
+    return {"DEFAULT_CARD_IMAGE": settings.DEFAULT_CARD_IMAGE}