[PMS] Include confirm attendance option for non-hackers

[taesungh]

Following from #322, we'll need to update the ParticipantAction button to allow confirming the attendance of a non-hacker.

• Update ParticipantAction to show Confirm button when participant's role is not applicant
  • Send POST request to endpoint from #322
• Disable the button when the user is not a director and show a corresponding tooltip/popover
  • See #340 and #346 for reference
• A modal would be nice but is probably unnecessary
  • Directors can notify tech if a confirmation needs to be undone for some reason