Utilize dormant user period in application settings

What does this MR do and why?

This is MR 2 of 2.

This expands upon the creation of the deactivate_dormant_users_period database column merged in !94383 (merged). It allows the User model to seek the value and also adds the UI to the Admin's settings page to set the value of the column in the database.

Relevant GitLab issue: #336747 (closed)

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