Ensure design notifications are sent

What does this MR do?

This MR fixes a bug preventing design discussion notifications from being sent.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

This notification setting respects the feature flags, so we will only start sending notifications once this is enabled.

This MR adds tests to verify that notifications are sent to the mailer.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

/cc @phikai

Fixes: https://gitlab.com/gitlab-org/gitlab-ee/issues/12527