Drone CI service

CHANGELOG

app/controllers/projects/services_controller.rb

app/models/project.rb

app/models/project_services/ci_service.rb

app/models/project_services/drone_ci_service.rb

app/models/service.rb

doc/api/services.md

doc/web_hooks/web_hooks.md

lib/api/helpers.rb

lib/api/services.rb

lib/gitlab/backend/grack_auth.rb

lib/tasks/services.rake

spec/models/project_services/drone_ci_service_spec.rb

spec/requests/api/services_spec.rb

spec/support/services_shared_context.rb