Allow all AutoDevOps banners to be disabled

Given the default in the development and production environment is
false, the negation of enabling is used in the flag to signal you'd turn
it off. It reads a bit awkward, but makes us have a migration less.

Fixes gitlab-org/gitlab-ce#37653

app/helpers/auto_devops_helper.rb

 module AutoDevopsHelper
   def show_auto_devops_callout?(project)
+    Feature.get(:auto_devops_banner_disabled).off? &&
       show_callout?('auto_devops_settings_dismissed') &&
       can?(current_user, :admin_pipeline, project) &&
       project.has_auto_devops_implicitly_disabled?

changelogs/unreleased/zj-feature-flipper-disable-banner.yml

+---
+title: Allow all AutoDevOps banners to be turned off
+merge_request:
+author:
+type: changed

doc/topics/autodevops/index.md

@@ -323,6 +323,23 @@ container registry. **Restarting a pod, scaling a service, or other actions whic
 require on-going access to the registry will fail**. On-going secure access is
 planned for a subsequent release.
 
+## Disable the banner instance wide
+
+If an administrater would like to disable the banners on an instance level, this
+feature can be disabled either through the console:
+
+```basb
+$ gitlab-rails console
+[1] pry(main)> Feature.get(:auto_devops_banner_disabled).disable
+=> true
+```
+
+Or through the HTTP API with the admin access token:
+
+```
+curl --data "value=true" --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/features/auto_devops_banner_disabled
+```
+
 ## Troubleshooting
 
 - Auto Build and Auto Test may fail in detecting your language/framework. There

spec/helpers/auto_devops_helper_spec.rb

@@ -10,6 +10,8 @@ describe AutoDevopsHelper do
     before do
       allow(helper).to receive(:can?).with(user, :admin_pipeline, project) { allowed }
       allow(helper).to receive(:current_user) { user }
+
+      Feature.get(:auto_devops_banner_disabled).disable
     end
 
     subject { helper.show_auto_devops_callout?(project) }
@@ -18,6 +20,14 @@ describe AutoDevopsHelper do
       it { is_expected.to eq(true) }
     end
 
+    context 'when the banner is disabled by feature flag' do
+      it 'allows the feature flag to disable' do
+        Feature.get(:auto_devops_banner_disabled).enable
+
+        expect(subject).to be(false)
+      end
+    end
+
     context 'when dismissed' do
       before do
         helper.request.cookies[:auto_devops_settings_dismissed] = 'true'