Feature/further request event fixes

The previous fix for #460, https://github.com/mysociety/alaveteli/commit/b24ad7c7fa9b12303cc3c558de6dbdaacfe501e2, does fix the problem of request events ending up with an incorrect latest_status value when a status update is made, and so turning up in the wrong results sets for status-based searches. But it has the disadvantage that setting the calculated state on the new 'status_update' event stopped the state propagating backwards to any preceding response. So searches that combined a 'variety:response' and status filter would have been missing requests. This pull request includes a fix that still allows the state to propagate backwards to any recent response, adds a similar fix for admin edits, and includes a script to fix up the event history of any requests that are currently returning an incorrect value for 'latest_status'.