**app/models/environment.rb:**
Adds a `cancel_deployment_jobs!` method which goes through and cancels
deployment jobs associated with that environment.

**app/services/merge_requests/base_service.rb:**
Adds a `cancel_deployment_jobs!` method that takes a merge request
and cancels deployment jobs for associated environments.

**app/services/merge_requests/post_merge_service.rb:**
Post merge we cancel all deployment jobs.