[FIX] website_mail_channel: fix load more replies

On a thread with at least 21 replies, click on "show N more replies".
The first time, it does the last 10 replies.
Click again: it appends the same batch of 10 replies to the thread.
The button loads the replies starting from the last reply id,
but this id is not updated after load.

This is because the code searches for a `p.well` in the DOM
assumed to be the `o_mg_read_more` parent (see c40f1841).
However, commit 59237ea8 removed the `.well` class of the parent.
The new button is not found, the old button is not removed, and so it goes.

addons/website_mail_channel/static/src/js/website_mail_channel.js

@@ -37,9 +37,9 @@ $(document).ready(function () {
             if ($thread_container) {
                 var $last_msg = $thread_container.find('li.media').last();
                 $(data).find('li.media').insertAfter($last_msg);
-                $(data).find('p.well').appendTo($thread_container);
+                $(data).find('.o_mg_read_more').parent().appendTo($thread_container);
             }
-            var $show_more = $link.parents('p.well').first();
+            var $show_more = $link.parent();
             $show_more.remove();
             return true;
         });