This commit adds some tests on mailing.activity to ensure _searching is only
possible when the user has access to the underlying document.
Query count for 'test_adv_activity_mixin' had to be slightly adapted (+1) since
calling 'action_close' in turn calls 'activity_search' that calls 'search' on
mail.activity that now runs an additional query (see '_search' override
docstring).
Task-2272475