Require attribution in OperationalImporterPscSync#{import, reset}

OperationalImporterPscSync is used in different responsibility contexts, and IMO there is no appropriate default.  (In the merge code, it's the user who submitted the candidate data; in the import tasks, it's run as a general "import" user.)  Under this assumption, there's two possibilities:

1. Keep the arity of `#import` and `#reset`, but raise an error if a responsible user is not set.
2. Change the arity of`#import` and `#reset`, which raises responsibility assignment to a syntax-level requirement. (Mostly.  A check for a non-nil responsible user might also be useful.)

In this PR, I chose approach 2.