Allow partners to override site header image

(the big one)

website/partners/templates/partners/partner.html

 {% extends "base.html" %}
 {% load i18n %}
 
+{% block header_image %}{% spaceless %}
+    {% if partner.site_header %}
+        {{ partner.site_header.url }}
+    {% else %}
+        {{ block.super }}
+    {% endif %}
+{% endspaceless %}{% endblock header_image %}
+
 {% block body %}
 
     <h1>{{ partner.name }}</h1>

website/thaliawebsite/templates/base.html

@@ -83,7 +83,10 @@
         <div id="header-panel" class="wrapper header-parallax">
 
             {# todo header image #}
-            <div class="header-image container" style="background-image: url('{% static "images/header_default.jpg" %}')"></div>
+            <div
+                class="header-image container"
+                style="background-image: url('{% block header_image %}{% static "images/header_default.jpg" %}{% endblock header_image%}')">
+            </div>
             <div class="page-title">
                 <h1>{% block page_title %}{% trans "Study Association Thalia" %}{% endblock %}</h1>
             </div>