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.
On system restart, an admin might want to bump this value temporarily, depending on how long samson is down, so as not to have the cron stop certain pending jobs.   Besides this, different installations might want different pending durations, and they should not have to locate code to change a constant.