Use sponsor logo from the newsletters app static, since this logo is always...

Use sponsor logo from the newsletters app static, since this logo is always correct (transparent + white) and the uploaded logo might not be.

website/newsletters/templates/newsletters/email.html

@@ -204,7 +204,7 @@
                 <td align="center" style="background-color: black; color: white;font-size: 12px;">
                     <p style="margin: 10px; font-family: 'Gill Sans', 'Trebuchet MS', sans-serif;">
                         {% if main_partner %}
-                            <a href="{{ main_partner.link }}"><img src="{% baseurl %}{{ main_partner.logo.url }}" height="70" style="max-width: 170px; height: 70px; margin-bottom: 5px" alt="{{ main_partner.name }}"/></a><br/>
+                            <a href="{{ main_partner.link }}"><img src="{% baseurl %}{% static 'newsletters/sponsor-logo.png' %}" height="70" style="max-width: 170px; height: 70px; margin-bottom: 5px" alt="{{ main_partner.name }}"/></a><br/>
                             {% trans "our main partner"|upper %}
                         {% endif %}
                     </p>