Add custom booklet headers/footers feature

The headers/footers are defined with the |customHeader| and
|customFooter| keys respectively in the concert JSON definition.

Change-Id: I461266ca8680a1b79abc85e004c92b810a4782fd
diff --git a/index.html b/index.html
index 48204f1..5eb4d21 100644
--- a/index.html
+++ b/index.html
@@ -34,11 +34,12 @@
       </section>
       <section id="booklet" class="booklet" hidden>
         <div class="booklet-header">
-          <h2 id="concert-title" class="booklet-header-title">Concert de Nadal 2020</h2>
-          <div id="concert-subtitle" class="booklet-header-subtitle">Joves i adults</div>
+          <h2 id="concert-title" class="booklet-header-title"></h2>
+          <div id="concert-subtitle" class="booklet-header-subtitle"></div>
         </div>
-        <div id="booklet-content" class="booklet-content">
-        </div>
+        <div id="booklet-custom-header" class="booklet-custom-header"></div>
+        <div id="booklet-content" class="booklet-content"></div>
+        <div id="booklet-custom-footer" class="booklet-custom-footer"></div>
       </section>
     </div>
     <div id="update-available" class="update-available" hidden>