[FIX] mail: link in chat messages

Commit f30ec302 defines the messages sent thanks to the channels as
plain text. This is true for all cases but one: the notification
(e.g. @user). Therefore, the notifications links are broken.

opw-687781

addons/mail/static/src/js/chat_manager.js

@@ -712,7 +712,7 @@ var chat_manager = {
             // post a message in a channel
             return ChannelModel.call('message_post', [options.channel_id], _.extend(msg, {
                 message_type: 'comment',
-                content_subtype: 'plaintext',
+                content_subtype: 'html',
                 subtype: 'mail.mt_comment',
             }));
         }