[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