Allow admins to edit email addresses of mailing list accounts again

Summary:
rP04b9f946020de32c513b69954933630641e8ff8f introduced `isEditableByAdministrators()` in `src/applications/settings/panel/PhabricatorSettingsPanel.php`.
In rP421bf2e548f06411a2ce86583333ec5e3e92c643, `isEditableByAdministrators()` was renamed to `isManagementPanel()`.
Thus the addition of `isEditableByAdministrators()` to `src/applications/settings/panel/PhabricatorEmailAddressesSettingsPanel.php` in rPe0a3e6ba3b87f96c83905e7188bb6161f7480784 had no effect - that function has no callers anywhere in the codebase.
Fix that by using the correct function name.

Closes T16110

Test Plan:
* As an admin, go to http://phorge.localhost/settings/user/SomeMailingListUser/, see new item "Email Addresses" in the panel on the left.
* As an admin, go to http://phorge.localhost/settings/user/SomeMailingListUser/page/email/ directly, don't get a 404 anymore.
* As a non-admin, go to http://phorge.localhost/settings/user/SomeMailingListUser/page/email/ and get a 404 as before.
* As an admin, on http://phorge.localhost/settings/user/SomeMailingListUser/page/email/ directly, add a new email address, mark it as primary, be able to remove the old email address. Check the `phabricator_user.user_email` DB table that the account's email address has been correctly updated.

Reviewers: O1 Blessed Committers, mainframe98, valerio.bozzolan

Reviewed By: O1 Blessed Committers, mainframe98, valerio.bozzolan

Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16110

Differential Revision: https://we.phorge.it/D26087