Create admin endpoint to get users that attended an event

[alexanderqchen]

Motivation

This will help with allowing officers to get a list of people who attended an event. This is a feature that has been requested a lot and has been fulfilled by manually running sql queries, so this will automate that process.

To Do

• add attendance objects to the dev-setup.js file to allow easy testing of attendance endpoints
• add admin authorization for attendance when uuid is included in endpoint

[alexanderqchen]

Hey so I just realized that this endpoint already exists here. But it is currently accessible by non-admins. I updated the description with what I want you to do now.