Fix EZP-21035: User can not be properly created from inside storage

This PR resolves issue https://jira.ez.no/browse/EZP-21035

Inability to create functional user from inside storage is solved by throwing BadStateException when adding user account type field definition to the ContentType that has Content instances. This is checked in storage as no way exists to check for content instances in BL.

Also added is BadStateException on adding user account type field definition to ContentType if it already contains one.