Experimental - Add buddy check system

### This is an experimental implementation :no_entry: 
- This PR adds buddy system - it uses the existing confirmation setting on the stage to determine whether it should get triggered, we probably want something else to do this job.
- The buddy check endpoint & the button has not been guarded by a check of whether it's the same deployer, it's for the ease of local testing.
- Tests not fixed

/cc @zendesk/samson @jwswj @pswadi-zendesk 
You probably want to move the implementation into the model itself, e.g.

```ruby
# app/models/deploy.rb
def confirm_buddy!(name)
  update_attribute(:buddy, current_user.name)
end
```