When rollout status is not yet complete, set Poll-Interval header

This way the frontend knows how often to poll for changes.

app/controllers/projects/environments_controller.rb

@@ -132,6 +132,8 @@ class Projects::EnvironmentsController < Projects::ApplicationController
 
     rollout_status = @environment.rollout_status
 
+    Gitlab::PollingInterval.set_header(response, interval: 3000) unless rollout_status.try(:complete?)
+
     if rollout_status.nil?
       render body: nil, status: 204 # no result yet
     else

spec/controllers/projects/environments_controller_spec.rb

@@ -266,6 +266,7 @@ describe Projects::EnvironmentsController do
         get :status, environment_params
 
         expect(response.status).to eq(204)
+        expect(response.headers['Poll-Interval']).to eq(3000)
       end
 
       it 'returns the rollout status when present' do