Show check-ins per day in Participants table

[taesungh]

As part of #344.

• Provide checkins list in participants endpoint
• Add columns for each day of the event with an icon on check-in status
• Disable resizable columns

Testing

1. Add sample applicant records with ATTENDING status if you don't have them already
2. Load the Participants page on the Admin site
3. Observe the check-in icons are all pending
4. Check in an applicant
5. Refocus the page to trigger a refetch (since we don't have the mutation yet)
6. Observe the icon for Friday has changed to positive

[github-actions[bot]]

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