Add API for setting registration date on items

[Nicklas Nordborg]

The registration date is currently set automatically by the core when new items are created. This makes it impossible to copy data between servers while preserving the registration date from the original server.

We should add an API method to the Registered interface that can set the registration date. It should only be possible to set on newly created items before they are saved to the database (the registration date column is mapped with update=false in Hibernate).

The batch importers should be able to import the registration date from data files. See #2062.