Default moderator is removed when creating an event via non-admin account

[emilghittasv]

Preconditions
Sign in to Moderator with a non-admin account.

Steps to reproduce
Steps to reproduce the behavior:

1. Access the https://moderator.allizom.org/event/new page.
2. Add some valid data inside the 'Event title' and 'Additional information' fields.
3. Click on the 'save' button.
4. Click on the 'edit' button for the newly created event.

Expected behavior
The default moderator (the non-admin account from the preconditions) is displayed/added inside the Moderators field.

Actual behavior
The Moderators field is empty.

Screencasts

Additional context
Maybe related to #432
This issue is not reproducible if the above steps are performed while signed in with an admin account.

[akatsoulas]

Closing as duplicate of #437