[lib] move isStaff() to separate file

Summary:
This is done to avoid acyclic imports after adding the validators. Without this shared worker is failing:

Test Plan: Run db on web (see rest of the stack) and check if works without errors.

Reviewers: tomek, michal

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7738