Downstream changes and extract email sender domain

I'm sick of having a private version of Samson. This is an attempt to bring anything from our private version of Samson which should be in the public version and extract a few extra configuration options to ENV variables

### TODO
- [x] Squash the history on this branch

cc /@zendesk/samson
I had to make [this commit](https://github.com/zendesk/zendesk_samson/commit/1dbdeff3a9955191bd8575f212e65d25b600efdd) for some reason, I imagine it will have to be the same here