Experimental - Add buddy check system

### This is an experimental implementation
- 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.

/cc @zendesk/samson @jwswj @pswadi-zendesk