Enhance the extension install modal dialog

lib/Vroom/I18N/en.pm

@@ -51,6 +51,7 @@ our %Lexicon = (
   "CANT_SHARE_SCREEN"                    => "Sorry, your configuration does not allow screen sharing",
   "SCREEN_SHARING_ONLY_FOR_CHROME"       => "Sorry, but you can't share your screen. Only Google Chrome supports this feature for now",
   "SCREEN_SHARING_CANCELLED"             => "Screen sharing has been cancelled",
+  "EXTENSION_REQUIRED"                   => "An extension is required",
   "VROOM_CHROME_EXTENSION"               => "To enable screen sharing, you need to install an extension. Click on the following link and refresh this page",
   "EVERYONE_CAN_SEE_YOUR_SCREEN"         => "All other participants can see your screen now",
   "SCREEN_UNSHARED"                      => "You do no longer share your screen",

lib/Vroom/I18N/fr.pm

@@ -57,6 +57,7 @@ our %Lexicon = (
     "SCREEN_SHARING_ONLY_FOR_CHROME"            => "Désolé, mais vous ne pouvez pas partager votre écran. Seul le navigateur Google Chrome " .
                                                    "supporte cette fonction pour l'instant",
     "SCREEN_SHARING_CANCELLED"                  => "Le partage d'écran a été annulé",
+    "EXTENSION_REQUIRED"                        => "Une extension est nécessaire",
     "VROOM_CHROME_EXTENSION"                    => "Pour activer le partage d'écran, vous devez installer une extension, cliquez sur le lien ci-dessous, puis raffraîchissez cette page",
     "EVERYONE_CAN_SEE_YOUR_SCREEN"              => "Tous les autres participants peuvent voir votre écran",
     "SCREEN_UNSHARED"                           => "Vous ne partagez plus votre écran",

templates/default/join.html.ep

@@ -72,15 +72,23 @@
     </nav>
     <div class="modal fade" role="dialog" id="chromeExtMessage" aria-labelledby="chromeExtMessage" aria-hidden="true">
       <div class="modal-dialog modal-sm">
-        <div class="modal-content well">
+        <div class="modal-content">
+          <div class="modal-header">
+            <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
+            <h4 class="modal-title" id="chromeExtMessageTitle"><%=l 'EXTENSION_REQUIRED' %></h4>
+          </div>
+          <div clas="modal-body">
             <p><%=l 'VROOM_CHROME_EXTENSION' %></p>
             <a href="https://chrome.google.com/webstore/detail/<%= $config->{chromeExtensionId} %>" target="_blank">
+              <h1>
                 <span class="glyphicon glyphicon-download-alt">
                 </span>
+              </h1>
             </a>
           </div>
         </div>
       </div>
+    </div>
     <div class="modal fade" role="dialog" id="noWebrtcSupport" aria-labelledby="noWebrtcSupport" aria-hidden="true">
       <div class="modal-dialog modal-lg">
         <div class="modal-content">