Fix group membership publicity

* Read the new flag in the summary API (the one we were reading
   was actually whether the group server listed you as a member to
   non-members).
 * Remove call to now-dead _loadGroupFromServer andf use the store
   instead

src/stores/GroupSummaryStore.js

@@ -68,4 +68,10 @@ export default class GroupSummaryStore extends EventEmitter {
             .removeUserFromGroupSummary(this._groupId, userId)
             .then(this._fetchSummary.bind(this));
     }
+
+    setGroupPublicity(is_published) {
+        return this._matrixClient
+            .setGroupPublicity(this._groupId, is_published)
+            .then(this._fetchSummary.bind(this));
+    }
 }