238876-support-deployment-events-webhook-at-group-level.yml

We recently released deployment webhooks on the project level,
but have forgotten about group interface for them.

Interface re-uses the same view, but the parameter is not
whitelisted for group-webhooks.

So this commit:
- adds "deployment_events" to triggerable events for group
- adds it to controller whitelist(and refactors it a little)
- adds it the API params and response
- adds it to group hooks API docs