[FIX] mail: display block for message body

The message body must be inside a tag div because when the message was short
with a @author, there was no tag div around the body and it was displayed
inine.

opw:666840

addons/mail/static/src/xml/thread.xml

@@ -122,7 +122,7 @@
                         </div>
                     </t>
                     <t t-if="!options.shorten_messages || !message.body_short">
-                        <t t-raw="message.body"/>
+                        <div><t t-raw="message.body"/></div>
                     </t>
                 </t>
                 <t t-foreach="message.attachment_ids" t-as="attachment">