AO3 6846 Add admin_hidden_work_notification mailer

[indesotw]

Pull Request Checklist

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• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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Issue

https://otwarchive.atlassian.net/browse/AO3-6846

Purpose

• Adds admin_hidden_work_notification mailer
• Removes html_safe calls in HTML mailer

Testing Instructions

See Jira

References

N/A

Credit

Indes (they/he)

[Bilka2]

Thank you!