Use an event's category image for OpenGraph image when available

get_together/templates/get_together/events/show_event.html

@@ -8,7 +8,11 @@
   <meta property="og:type"          content="website" />
   <meta property="og:title"         content="{{event.name}}" />
   <meta property="og:description"   content="{{event.summary}}" />
+  {% if event.team.category %}
+  <meta property="og:image"         content="{{event.team.category.img_url}}" />
+  {% else %}
   <meta property="og:image"         content="https://gettogether.community{% static 'img/team_placeholder.png' %}" />
+  {% endif %}
 {% endblock %}
 
 {% block styles %}