DEB-1333 Added support for child default language.

Had to implement a bit of a hack, since setting the default language is not supported by the API endpoint.

What is done is that after creating the patron with guardian, the created patron is fetched, and an update request is run to set the default language.

This means to additional API calls in user creation time. If any of these fail, the registration itself does not fail, only the default language does not get set.

Updated Patron creation endpoints, and default comm. channel settings, as agreed with Marie.