Use inspect_table; default user license==None.

Use inspect_table; default user license==None.

Use inspect_table in the new migration. Makes code more
readable, really.

And make the default for the preferred license be None.
This is a userspace thing, so we can even change the
migration here. Changing the migration means, that people
running the migration before this commit get a "" in
User.license_preference, while people running the migration
now get a None. Both values are okay.

None has been designated as "Use the site's default". We're
not actually having a site default right now. Which means
no license is selected in the dropdown.

While "" means "All rights reserved" being chosen by the
user.

Side note: Having no license being selected in the submit
dropdown is as "worse" as before and does not really hurt
much. MediaEntry.license==None means "All rights reserved"
as does "" also do.