Fix EZP-20681: Not able to login with a user created via REST

This PR fixes issue https://jira.ez.no/browse/EZP-20681

The reason for bug was SPI cache for corresponding Content was not invalidated after creation of User, thus loaded user had no external data for ezuser Field set. This showed up only when loading the created User, but not in the User object returned by ```UserService::createUser()``` because there the SPI User object returned by ```UserHandler::create()``` is used to build domain object.

Additionally: removed the need for reloading User Content in ```UserService::createUser()```. SPI cache still being hit the same though.