Fixed duplicated messages sending in slow network condition.

Show the message sending flow state in the messages list:
  - While sending, the message appears semi transparent in the chat.
  - If successfully sent, it appears as before, ie normal
  - In case of failure, it appears in red with an Unsent text.

webclient/app.css

@@ -417,6 +417,13 @@ a:active  { color: #000; }
     text-align: left ! important;
 }
 
+.bubble .messagePending {
+    opacity: 0.3
+}
+.messageUnSent {
+    color: #F00;
+}
+
 #room-fullscreen-image {
     position: absolute;
     top: 0px;