Add a temporary Summit CFP announcement banner

Link to the call for presentations site for the upcoming OpenInfra
Summit with the optional banner mechanism we've used in the past.
Please don't merge this until 15:00 UTC on 2022-11-15, since the
site won't have CFP info until that time.

This can be reverted after the CFP window closes on 2023-01-10.

This partly reverts commit 5ceda92172.

Change-Id: Icef393a05b1930d88ff16fbd0674b7e457c51846

www/assets/css/main.css

@@ -1930,13 +1930,21 @@ input[type="button"],
   height: 28em;
   text-align: center;
   position: relative; }
-  #banner header {
-    position: absolute;
-    bottom: 30%;
-    left: 0;
+  #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; }
+    #banner p .button {
+      margin-left: 1em;
+      margin-top: 1em; }
+  #banner header {
     color: #071d49;
-    padding: 1.5em 0; }
+    margin: 6%; }
     #banner header h2 {
       display: inline-block;
       margin: 0;

www/index.html

@@ -72,6 +72,17 @@
 
       <!-- Banner -->
         <section id="banner">
+          <p>
+            <strong>OpenInfra Summit Vancouver Call for Presentations is now
+                Open!</strong>
+            <br/>
+            The OpenInfra Summit Vancouver is the Zuul community’s chance to
+            highlight the open source innovation around Project Gating and
+            CI/CD.
+            <br/>
+            <a href="https://www.openinfra.dev/summit/vancouver-2023"
+              class="button">Submit Your Presentation</a>
+          </p>
           <header>
             <h2>Stop Merging Broken Code</h2>
           </header>