non-admin accounts are not able to remove users from the "Moderators" list

[emilghittasv]

Preconditions
Sign in to https://moderator.allizom.org with an admin account.

Steps to reproduce
Steps to reproduce the behavior:

1. Access the https://moderator.allizom.org/event/new page.
2. Add valid data inside the input fields.
3. Add a non-admin account inside the "Moderators" field and create the event.
4. Sign in to https://moderator.allizom.org using the non-admin account used in step 3.
5. Access the posted event from step 3.
6. Delete a username from the "Moderators" field and click the "save" button.
7. Access the the posted event again.

Expected behavior
The username removed in step 6 is no longer displayed inside the "Moderators" field.

Actual behavior
The username which was removed in step 6 is still displayed inside the "Moderators" field.

Screencasts

Additional context
This issue is not reproducible when removing the "Moderators" while signed in with an admin account.
Maybe also related to #432

[akatsoulas]

Closing as duplicate of #437