Populate an SA's responsible user.  #3662.

This pull request contains a prerequisite for https://code.bioinformatics.northwestern.edu/issues/issues/show/3662.  In particular, this lays the foundation for event selection by data collector.

The only basis I have for the activity mappings established in this commit is "well, they're the only usernames I see in the activity JSON", which doesn't really mean anything.  Please review whether or not they're correct.

Additionally, I would like a word on whether or not the `if assign['subject_coordinator']` is actually required.  Some test data omits the assignment.subject_coordinator block, but it's possible that the test data is wrong, and I haven't yet dug into PSC's code or API docs to see what guarantees it provides.