R1:478012cd463d

Add serious business string for defrocking a user (remove admin rights)

Summary:
Allow humans to understand what happened when `phabricator.serious-business` mode is enabled: The administrator role was removed from a user account.

Closes T15480

Test Plan: Enable `phabricator.serious-business` mode. Enable and disable an account as admin. Check that account's feed and the general feed.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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