Restructure audiovisual file data structure for UUIDs

[camdendotlol]

Summary

• changes the format of the audiovisual_files array in event objects to an object with UUIDs as keys (see below)
• updates the EventForm and a couple other places to work with the new structure

Example

Old format

type Event = {
  audiovisual_files: {
    label: string;
    file_url: string;
    duration: number;
  }[]
  // ...
}

New format

type Event = {
    audiovisual_files: {
      [key: string]: {
        label: string;
        file_url: string;
        duration: number
      }
    }
  // ...
}