Allow silos to not implement Listen

Although they will be in the minority, there is the chance that we may
not support Listen flows on some silos.

Therefore we can make sure that `social_user` keeps this in mind when
rendering, and ignore the Listen-specific UI if unnecessary.

models.py

@@ -85,6 +85,8 @@ class Source(StringIdModel, metaclass=SourceMeta):
   GR_CLASS = None
   # oauth-dropins StartHandler class
   OAUTH_START_HANDLER = None
+  # whether Bridgy supports listen for this silo - this is unlikely, so we default to True
+  CAN_LISTEN = True
   # whether Bridgy supports publish for this silo
   CAN_PUBLISH = None
   # how often to poll for responses

templates/social_user.html

@@ -56,6 +56,7 @@
 
 <div class="row">
 
+{% if source.CAN_LISTEN %}
 <!-- Listen UI -->
 <div id="listen-ui" class="{% if source.CAN_PUBLISH %}col-sm-6{% endif %}">
 
@@ -179,10 +180,11 @@
 {% endif %}
 
 </div>
+{% endif %}
 
 
 <!-- Publish UI -->
-<div id="publish-ui" class="col-sm-6">
+<div id="publish-ui" class="{% if source.CAN_LISTEN %}col-sm-6{% endif %}">
 
 {% if "publish" in source.features and source.status != "disabled" %}
 <data class="p-bridgy-publish-status" value="enabled">
@@ -329,7 +331,7 @@
   {% endfor %}
 </ul>
 
-{% else %}
+{% elif source.CAN_LISTEN %}
 <p class="big">No responses.</p>
 {% endif %}