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
- [x] 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.
It was not clear at the time it was added that the JIRA email would not need something additional in the subject or elsewhere, to trigger the ticket creation.  For this reason - and also not wishing to change code after a review yesterday - I added a full additional mailer section, with the understanding that refactoring could be done later.