[FIX] mail: hidden chatter tooltip can cover content

With 72dfcae2a we have an envelope icon in the chatter with the
information on the sent mail.

When not displayed, the tooltip is hidden using opacity:0 so if
something is underneath it it becomes unclickable.

This commit makes the tooltip disappear in this instance.

opw-705205

addons/mail/static/src/less/thread.less

@@ -177,6 +177,7 @@
                     border-radius: 5px;
                     color: gray;
                     width: 300px;
+                    display: none;
                     opacity: 0;
                     position: absolute;
                     transition: opacity 0.5s;
@@ -186,6 +187,7 @@
                 }
 
                 .o_thread_tooltip:hover + .o_thread_tooltip_content {
+                    display: inline;
                     opacity: 1;
                 }
             }