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

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15480

Differential Revision: https://we.phorge.it/D25298