Add workaround when inserting new fields to form field list

i.e. if inserting "before" or "after" a field which does not exist

tailbone/views/users.py

@@ -69,6 +69,7 @@ class UserView(PrincipalMasterView):
         'active_sticky',
         'set_password',
         'roles',
+        'permissions',
     ]
 
     row_grid_columns = [
@@ -256,11 +257,12 @@ class UserView(PrincipalMasterView):
 
         if self.viewing:
             permissions = self.request.registry.settings.get('tailbone_permissions', {})
-            f.append('permissions')
             f.set_renderer('permissions', PermissionsRenderer(request=self.request,
                                                               permissions=permissions,
                                                               include_guest=True,
                                                               include_authenticated=True))
+        else:
+            f.remove('permissions')
 
         if self.viewing or self.deleting:
             f.remove('set_password')