Revert "Remove userId from the list of editable-by-default fields"

This reverts commit 4fe60b6feed568a6cfdca2b6aabe80883303cd80.
ZfcUser 1.x uses the default ClassMethods hydrator, resulting in 'id' instead of 'userId' when the entity is extracted
(naturally this causes havok with updating user records via the provided forms; this fixes that)
Implement NoOtherRecordExists validator to enforce uniqueness of username and email fields