Add buddy check system

This adds a feature to Samson that can make samson require a Buddy for certain stages. The use case here is when deploying to production stages you want to have at least 2 developers know what code is being shipped.

/cc @zendesk/samson @jwswj @pswadi-zendesk 

### TODO

- [x] fix old tests
- [ ] add new tests
- [x] add email notification for `bypass`

### RISKS
- still using the confirmation setting on stages to determine whether a stage needs buddy check.
- If this branch gets merged, the measurement of the deployment duration needs to be fixed.
Yes, there is already a differentiating prefix for the Subject in deploy_mailer.rb for bypass_alert(); the subject for deploy_email() remains as it is, *without* a prefix.

Last Friday you asked for additional differentiation, so I added it to the summary text, and I made those changes where the summary text is assembled, which is model/deploys.rb.

If you think the Subject differentiation is sufficient, I can remove it from model/deploys.rb