[PMS] Incorporate standard check-in functionality into Participants page

[taesungh]

For our standard check-in, we'll need to connect the Check In button with an API call to the check-in endpoint. This should open a modal with a confirmation step including instructions on the standard procedure (check photo ID, sign SPFB sheet, etc.) and will eventually include a step to scan or enter a participant's badge number as part of #338.

• [x] Open a Modal when using the Check In button
• [x] Show instructions for check-in associate in modal and action to confirm check-in
• [x] Send a POST request to the check-in endpoint once confirmed through the modal
• [ ] Show a notification with a Flashbar when check-in succeeds