Participant import issues

A couple of issues with participant import:

1. The docs say that Participant Role is required, while a role-less import works (role ends up being Attendee).
2. An attempt to import with status Attended ends up with the status being set to Registered.

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1. Fix onscreen help text (done) to say that participant role is not required, and the event default role will be set if role isn't passed.
2. Fix status import so it assigns the proper status.