Add "Attendees" field to drupal.org Event

Problem/Motivation

The ability for an event organizer to credit an event's attendees would:

• Give attendees an immediate reason to create a Drupal.org account
• Provide a (mostly symbolic) incentive for attendees to show up to an event

Both of these support community growth and engagement for those new to the community.

Original Slack thread

Proposed resolution

• Add "Attendees" field to drupal.org Event (analogous to Code of conduct contact, Organizers, Speakers, and Volunteers)
• Document the purpose of this field, why an organizer should consider doing the work to collect drupal.org usernames from attendees, and how to enter many usernames quickly.
• Possibly result in an "Events attended" badge on user profiles

Remaining tasks

User interface changes

API changes

Data model changes