[FIX] mail: link in livechat

When a user sends a link thanks to the livechat, the link is not
clickable is the recipient is on the frontend (e.g. visitor of the
website). On the other hand, a link sent from the frontend to the
backend works.

This is because the message is sent as "html" from the backend,
therefore no link conversion is done during the post-process of the
massege (method `message_post`).

Note that a link sent and received in the backend works because
additional formatting is done (convert emojis, add links...).

opw-686063

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: 'html',
+                content_subtype: 'plaintext',
                 subtype: 'mail.mt_comment',
             }));
         }