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. For this `countInstancesOfContentType()` is added to the SPI ContentType handler, so that the check can be implemented in BL.

Also, method `isRepeatable()` is added to the SPI and API FieldType interfaces, returning boolean indicating if the field definition of the type can appear multiple times in the same ContentType. By default FieldType will be repeatable.
If field definition of the same non-repeatable type is being added to the ContentType already containing one, BadStateException will be thrown.

`ezuser` is marked as non-repetable, therefore it is not possible to add two field definitions of that type to the same ContentType.

### TODO
- [x] add `countInstancesOfContentType( $contentTypeId )` to the SPI Content handler interface and move the check if ContentType that has Content instances to the BL