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.
```
to_email = [BuddyCheck.bypass_email_address]
to_email << BuddyCheck.bypass_jira_email_address if BuddyCheck.jira_email_required?
```
is much nicer to read