[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:
Possibly, I can't tell for sure though because I don't know what `Discourse.base_url` actually depends on...

My point is that, if our intention of the deprecation is to warn people "you probably don't want this (using paths in mailers)" to protect 90% of the users, we should provide an easy alternative for those who want to say "yes I am sure and I have a valid reason for using paths" to do that explicitly.

Allowing `*_url(..., only_path: true)` without a `host` option would work for me, if it's feasible. Not sure why we are still complaining about the missing `host` when we know that we are only generating a path?