[FIX] im_livechat: match_rules: backward-compatibility

Add a dummy 'match_rules()' function for backward-compatibility of commit
d8605b79, as 'match_rules()' was called from  server-side 'im_livechat.loader'
template.

addons/im_livechat/models/im_livechat_channel.py

@@ -200,6 +200,10 @@ class ImLivechatChannel(models.Model):
             info['options']["default_username"] = username
         return info
 
+    # FIXME: to remove in master
+    @api.model
+    def match_rules(self, request, channel_id, username='Visitor'):
+        pass
 
 class ImLivechatChannelRule(models.Model):
     """ Channel Rules