At the moment, it's possible to have maximum 5 assignments per topic
(that includes topic and post assignments). When trying to assign more,
a message "Limit of 5 assignments per topic has been reached" appears.
One possible edge case here is _reassigning_ a topic or a post. Reassignment
doesn't lead to exceeding the limit, and therefore it should be possible. But
at the moment we handle correctly only _topic_ reassignments, while when
reassigning a _post_, we show the error message "the limit has been reached".
This commit makes post reassignments work correctly too.
51920a99c0