Marking Volunteers as Attended from the RSVP List Wipes Current Volunteer Names Fix

[JohnCox2211]

Fixes issue #1692

Changes:
When you mark volunteers as attended from the 'RSVP and Waitlist', they will now be added to the attended list and will still have their 'Attended' checkbox checked. Adding this change also fixed users disappearing from the 'Attended' table when different users are checked in the 'RSVP and Waitlist' table.

Testing:
Create Event:

• Create an event and make sure you toggle 'This event requires RSVP'
• The event should be upcoming so that you can RSVP volunteers to move them between tables
• Add RSVP volunteers
• When the event starts, you can move them between boxes with the 'Attended' toggle

Example:

You can freely add/remove volunteers from the 'Attended' table with the checkbox now without wiping the table, and the checkboxes are consistent once you hit save.

[Kafui123]

When I rsvp'd some students and the event started, they automatically showed up in the attended table without me clicking on the attended table.

[github-actions]

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[bakobagassas]

The UI and code look good now! Good Job!