When PX closes a suggestion that was actually implemented, ticking the
checkbox sends a high-value 'Good news — your suggestion was implemented'
SMS instead of the generic closed message. The flag is ignored for any
status other than CLOSED. Acknowledge-on-submit SMS already existed; this
completes the closed-loop value path.
The checkbox appears in the status-change form in feedback_detail.html.
Three tests cover: implemented flag on close, flag off on close, flag
ignored when transitioning to a non-closed status.
