[WIP] 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
- [ ] Squash the history on this branch

cc /@zendesk/samson
This should have been removed in fce5059, probably a bad merge