[FIX] mail: display chatter inside the sheet

Before this revision, it was not possible to display a chatter inside a <sheet>.

As the chatter is reused (when switching in edit, records, etc.), it is detached
(to keep its handlers) before updating the form $el.

The problem here is that the chatter was detached in `_updateView` from
`$form.contents()` where the chatter is a root element ; when setting the form
view html, the chatter was thus still present. This was thus only working
correctly if the chatter was a direct child of <form>.

To keep it displayed in all cases, we now detach it at the chatter node
rendering.

Closes #20032