[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
Yeah, we had noticed the change in public repo a while back. We want to keep the code that matches the email id and updates the same entry rather than create a new one.