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.
I have viewed these emails only in the log file, and from there I see there is already a subject line that has [BYPASS][DEPLOY] in it, as opposed to simply [DEPLOY].  Nonetheless, I added [BYPASS][DEPLOY] to the content as well for html and text.  Check-in is fd4c348 below.