Admin: new report edit pages unnecessarily ask for authentication

[abibroom]

After logging in to the FixMyStreet admin, the first time I click on an 'edit' link for a report causes my browser to prompt for username and password again (but it happily loads the page behind the dialogue box, and doesn't care if I dismiss the prompt without signing in). Not a browser issue - happens in Firefox 47 and Chrome 49 - and undoubtedly related to the new report editing pages as it only started happening when they went live.