Add a temporary summit promotional message

During lead up to the OpenStack Summit in Berlin, display a message
letting readers know that the Zuul team will be represented along
with some related talks/sessions, and include a link to additional
information about the event.

Change-Id: I7e58aaff69bd4425c0f3b32200907c036f8110f1

www/assets/css/main.css

@@ -1881,6 +1881,15 @@ input[type="button"],
   height: 28em;
   text-align: center;
   position: relative; }
+  #banner p {
+    display: inline-block;
+    margin: 0;
+    width: 100%;
+    vertical-align: middle;
+    background: #212121;
+    background: rgba(27, 27, 27, 0.75);
+    color: #37c0fb;
+    padding: 1.5em 0; }
   #banner header {
     position: absolute;
     bottom: 30%;

www/index.html

@@ -53,6 +53,12 @@
 
       <!-- Banner -->
         <section id="banner">
+          <p>
+            <strong>Meet the Zuul Community at the OpenStack Summit, November
+            13-15 in Berlin, Germany!</strong>
+            <a href="https://www.openstack.org/summit/"
+              class="button small">Learn More</a>
+          </p>
           <header>
             <h2>Stop Merging Broken Code</h2>
           </header>