rename column on app settings

[emilia-friedberg]

WHAT

Change enabled_for_admins to enabled_for_staff in both name and function.

WHY

When I started working on implementing the frontend feature that utilizes this code, I realized that enabled_for_admins was probably meant to turn on features for Quill admins (ie, staff members), not district admins. I checked with @dandrabik and he confirmed that this was correct, so I updated both the functionality and the naming to hopefully be clearer.

HOW

Add a migration to change the name, change everywhere we reference that column, and change the logic to check for staff members and not admins.

Screenshots

(If applicable. Also, please censor any sensitive data)

Notion Card Links

https://www.notion.so/quill/Change-enabled_for_admins-to-enabled_for_staff-on-App-Settings-c66650b28e6d456e88d1ec4b9f46f7b1

PR Checklist	Your Answer
Have you added and/or updated tests?	YES
Have you deployed to Staging?	NO - non-app change
Self-Review: Have you done an initial self-review of the code below on Github?	YES
Design Review: If applicable, have you compared the coded design to the mockups?	N/A

[dandrabik]

👍 LGTM! Thanks for making this change.