ISTWCMS-3661: adding placeholder for facebook block

source/_patterns/04-components/facebook/_facebook.scss

-.uw-facebook {
-  width: 100%;
-
-  &__wrapper {
-    margin-left: auto;
-    margin-right: auto;
-    text-align: center;
-  }
-}
+.uw-facebook {
+  width: 100%;
+
+  &__wrapper {
+    margin-left: auto;
+    margin-right: auto;
+    text-align: center;
+  }
+}
+
+.uw-facebook {
+  .fb-post,
+  .fb-page {
+    background: url('../source/images/icons/facebook_logo.png') no-repeat;
+    background-position: center center;
+    min-height: 340px;
+    text-align: center;
+    width: 100%;
+}

source/_patterns/04-components/facebook/facebook.twig

 <div class="uw-facebook">
   <div class="uw-facebook__wrapper">
-    <script async defer src="https://connect.facebook.net/en_US/sdk.js#xfbml=1&version=v3.2"></script>
     {% if feed_type == 'timeline' %}
       <div class="fb-page"
            data-href="https://www.facebook.com/{{ username }}"
@@ -8,13 +7,12 @@
            data-width="500"
            data-hide-cover="false"
            data-show-facepile="false"
-           adapt_container_width="true">
-      </div>
+           adapt_container_width="true"></div>
     {% else %}
       <div class="fb-post"
          data-href="{{ url }}"
-         data-width="500">
-      </div>
+         data-width="500"></div>
     {% endif %}
+    <script src="https://connect.facebook.net/en_US/sdk.js#xfbml=1&version=v3.2"></script>
   </div>
 </div>