[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:
@pixeltrix Sorry I force-pushed and lost your comment. :( To recap, this change is needed because...

1. Using `email_unsubscribe_path` produces a deprecation warning
2. Using `email_unsubscribe_url` without the `host` option raises an error, even with `only_path: true`

As for *why* they want a path here, it's because that the host is dynamic, and to avoid duplicating `Discourse.base_url` in every call they extracted it into a helper which takes a patch and "glues" the host plus other formatting stuff onto it, which doesn't seem like an unreasonable thing to do.

cc @fxn @schneems @matthewd @sgrif again