Unify custom service desk name flags

* use single feature flag check both for setting and delivering of
  service desk emails
* use project scope for this flag

ee/app/controllers/ee/projects_controller.rb

@@ -11,7 +11,7 @@ module EE
       before_action :log_unarchive_audit_event, only: [:unarchive]
 
       before_action do
-        push_frontend_feature_flag(:service_desk_custom_address)
+        push_frontend_feature_flag(:service_desk_custom_address, @project)
       end
     end
 

ee/app/services/service_desk_settings/update_service.rb

@@ -5,7 +5,7 @@ module ServiceDeskSettings
     def execute
       settings = ServiceDeskSetting.safe_find_or_create_by!(project_id: project.id)
 
-      unless ::Feature.enabled?(:service_desk_custom_address)
+      unless ::Feature.enabled?(:service_desk_custom_address, project)
         params.delete(:project_key)
       end
 

ee/lib/gitlab/email/handler/ee/service_desk_handler.rb

@@ -69,7 +69,7 @@ module Gitlab
           end
 
           def valid_project_key?(project, slug)
-            project.present? && slug == project.full_path_slug && Feature.enabled?(:service_desk_email, project)
+            project.present? && slug == project.full_path_slug && Feature.enabled?(:service_desk_custom_address, project)
           end
 
           def create_issue!

ee/spec/lib/gitlab/email/handler/ee/service_desk_handler_spec.rb

@@ -181,9 +181,9 @@ describe Gitlab::Email::Handler::EE::ServiceDeskHandler do
           end
         end
 
-        context 'when service_desk_email feature is disabled' do
+        context 'when service_desk_custom_address feature is disabled' do
           before do
-            stub_feature_flags(service_desk_email: false)
+            stub_feature_flags(service_desk_custom_address: false)
           end
 
           it 'bounces the email' do