Demodulize model headers in emails

What does this MR do?

This makes our email notification controller able to handle models that have a namespace. Also, since this is meant to be very general code, we do not assume the existence of an id.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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

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

This is required for https://gitlab.com/gitlab-org/gitlab-ee/issues/12527