[API] Allow checking in other types of participants

[taesungh]

Following from #268, we added an endpoint to check in hackers, but we still need to be able to check in non-hackers. In conjunction with #322 as part of #318, we'll assume for now the status of non-hackers will be updated to CONFIRMED or ATTENDING (exact TBD), so the logic is almost the same as for non-hackers. The only additional check we should need to make is that the user role is not empty.

• Repurpose participant_manager.check_in_applicant to allow checking in non-hackers.
• Make sure the user's role is not empty during the database query
• Use the same existing status check (ATTENDING or CONFIRMED)