fix: use email address instead of username when looking up users

[shreddedbacon]

General Checklist

• [ ] Affected Issues have been mentioned in the Closing issues section
• [ ] Documentation has been written/updated
• [ ] PR title is ready for inclusion in changelog

Database Migrations

• [ ] If your PR contains a database migation, it MUST be the latest in date order alphabetically

Just fixes the underlying resolvers and models to use the email address correctly.

Closing issues

closes #3656