[WIP] Upgrade to rails master/4.2

I was going to send this when I have the complete set of patches required, but I noticed @SamSaffron has started working on it too, so we should probably share notes :smile:
Why would it? :) We are generating a URL here, so we should use the url helper (doing _path(... only_path: false) is deprecated in the commit I linked in the comment)