backoffice: display potential session messages in listing page (#35759)

tests/test_backoffice_pages.py

@@ -1234,6 +1234,7 @@ def test_backoffice_global_remove_action(pub):
     resp = resp.form.submit('button-action-1')
     resp = resp.follow()
     assert resp.request.url == 'http://example.net/backoffice/management/test-global-remove/'
+    assert 'The form has been deleted.' in resp.body
 
 def test_backoffice_submission_context(pub):
     user = create_user(pub)

wcs/backoffice/management.py

@@ -1370,6 +1370,7 @@ class FormPage(Directory):
         r = TemplateIO(html=True)
         r += htmltext('<div id="appbar">')
         r += htmltext('<h2>%s - %s</h2>') % (self.formdef.name, _('Listing'))
+        r += get_session().display_message()
         r += self.listing_top_actions()
         r += htmltext('</div>')
         r += table