feat: move Slack config to installation settings (#11548)

- enable Slack Configuration via InstallationConfig
- list Slack integration in super admin settings

spec/enterprise/services/enterprise/clearbit_lookup_service_spec.rb

@@ -49,6 +49,10 @@ RSpec.describe Enterprise::ClearbitLookupService do
     end
 
     context 'when Clearbit is not enabled' do
+      before do
+        GlobalConfig.clear_cache
+      end
+
       it 'returns nil without making an API call' do
         with_modified_env CLEARBIT_API_KEY: nil do
           expect(Net::HTTP).not_to receive(:start)