Allow the REST API to handle subscribing via a webhook

:warning: This PR is based on https://github.com/zendesk/radar/pull/90 and should be merged into that branch before merging that branch. I just thought it could be useful for seeing the changes.

/cc @zendesk/zendesk-radar


### Steps to merge
 - [ ] :+1: of the team

### Risks
 - medium:could break subscribe/unsubscribe for engine.io clients