Refactor upgrade websocket progress, so "anyone" can join in to see

now while an upgrade is executing, anyone with permission can "view"
the upgrade and see the same progress the executor is seeing

tailbone/views/master.py

@@ -1063,6 +1063,8 @@ class MasterView(View):
             'instance_deletable': self.deletable_instance(instance),
             'form': form,
         }
+        if self.executable:
+            context['instance_executable'] = self.executable_instance(instance)
         if hasattr(form, 'make_deform_form'):
             context['dform'] = form.make_deform_form()
 
@@ -1784,6 +1786,14 @@ class MasterView(View):
         elif importer.allow_create:
             return importer.create_object(key, host_data)
 
+    def executable_instance(self, instance):
+        """
+        Returns boolean indicating whether or not the given instance
+        can be considered "executable".  Returns ``True`` by default;
+        override as necessary.
+        """
+        return True
+
     def execute(self):
         """
         Execute an object.