Fix topic limiting in forums/id endpoint
Fix topic limiting in forums/id endpoint

The topics_limit parameter was already added to permitted_params,
but it was not used in any way.

This commit finalizes the implementation of the topics_limit.

scott/helpy__helpdesk_knowledgebase,scott/helpy__helpdesk_knowledgebase,helpyio/helpy,scott/helpy__helpdesk_knowledgebase,helpyio/helpy,helpyio/helpy,helpyio/helpy