[FIX] mail: correctly compute unread messages

Two problems occured:
 - when the user hadn't seen any messages of a channel, the seen_message_id
   was null and doing seen_message_id < msg.id to retrieve all messages
   received after the last one seen didn't return any message (null < x is
   always false)

 - messages sent by visitor on the livechat doesn't have any author_id, so,
   again, doing msg.author_id != partner_id is always false, and as a
   consequence, server-side, the messages sent by visitors were always
   considered as read

0 files changed, 0 insertions, 0 deletions