Refactor backend logic for storage limit notification

ee/app/helpers/ee/namespace_storage_limit_alert_helper.rb

ee/app/models/ee/namespace/storage/notification.rb

ee/app/services/ee/post_receive_service.rb

.../services/namespaces/check_excess_storage_size_service.rb

ee/app/services/namespaces/check_storage_size_service.rb

ee/app/views/shared/_namespace_storage_limit_alert.html.haml

...c/helpers/ee/namespace_storage_limit_alert_helper_spec.rb

ee/spec/models/ee/namespace/storage/notification_spec.rb

ee/spec/requests/groups/usage_quotas_spec.rb

ee/spec/services/ee/post_receive_service_spec.rb

...ices/namespaces/check_excess_storage_size_service_spec.rb

...ec/services/namespaces/check_storage_size_service_spec.rb

...trollers/namespace_storage_limit_alert_shared_examples.rb