[FIX] crm: Button "Convert to opportunity" for an existing opportuniy

When opening an opportunity from a notification, it always opens the default view
which is the lead view and then the button "Button "Convert to opportunity" was visible.

opw:792931

addons/crm/views/crm_lead_views.xml

@@ -44,7 +44,7 @@
             <form string="Leads Form">
                 <header>
                     <button name="%(crm.action_crm_lead2opportunity_partner)d" string="Convert to Opportunity" type="action"
-                            help="Convert to Opportunity" class="oe_highlight"/>
+                            help="Convert to Opportunity" class="oe_highlight" attrs="{'invisible': [('type', '=', 'opportunity')]}"/>
                 </header>
                 <sheet>
                     <div class="oe_button_box" name="button_box">