In admin, prevent ability to create impersonation token for internal user

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From https://gitlab.com/gitlab-org/manage/import-and-integrate/discussions/-/issues/112#note_2076937556:

As an admin, I can go to the instance settings > Users and I'll see the placeholder users appear in the list. While the possibility to impersonate a placeholder user is deactivated/disabled, there's an option to create an Impersonation Token.

Although the API appears to deny internal users https://gitlab.com/gitlab-org/manage/import-and-integrate/discussions/-/issues/112#note_2077999694 we should remove the ability to create an impersonation tokens for internal users as a defence in depth measure.