SImplify and fix preference name migration
@hansegucker The current_site
object behaves strange inside the migration, I have no idea why (the same code works jsut fine outside the migration).
Trying to fix it, I wondered why the migration even needs to be site-aware (and if being site-aware isn't even a bug in itself), and what the update_or_create
is supposed to do. Please look into my simplification.
Closes #35 (closed).