[POC] Add buddy check system

:no_entry: This is an experimental implementation :no_entry: 

/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.
A few things: (a) having all db columns present at all times simplifies things, (b) I know we need tests that simulate the feature ON and OFF, but my test-writing experience is, er, very thin.  I'll revert the schema first, and then work from there.