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 
the above two lines can be guarded by a check like
```ruby
if @deploy.started_by?(current_user)
  ...
end
```