[FIX] im_livechat: prevent starting multiple poll

bus.poll() should be a private function, as it initializes a new longpolling.
Only bus.start_polling() should be called as it prevents running multiple
polls concurrently.

On the website, bus.poll() was called each time the user clicked on the
livechat button, blocking the whole UI if the user clicked 6 times in a row on
that button, as only 6 RPCs can be done concurrently.

A consequence of this fix is that the poll isn't restarted if the user opens
a new livechat session just after closing one, so the messages sent through
that session are delayed (~50 seconds). To circumvent this situation, we simply
don't restore the livechat button after closing a livechat session (the button
re-appears anyway at the next click, or after a refresh).

addons/im_livechat/static/src/js/im_livechat.js

@@ -84,7 +84,7 @@ var LivechatButton = Widget.extend({
                 self.render_messages();
 
                 bus.add_channel(channel.uuid);
-                bus.poll();
+                bus.start_polling();
 
                 utils.set_cookie('im_livechat_session', JSON.stringify(channel), 60*60);
             }
@@ -119,7 +119,6 @@ var LivechatButton = Widget.extend({
 
     close_chat: function () {
         this.chat_window.destroy();
-        this.$el.show();
         utils.set_cookie('im_livechat_session', "", -1); // remove cookie
     },