[lib] Factor out useStaffAlert()

Summary:
We want to show staff alerts also in other places in the app so I factored it out to a hook in another file.
Related to: https://linear.app/comm/issue/ENG-9807/add-staff-alert-for-decryption-related-errors

Test Plan: Run the app and see if alerts are shown.

Reviewers: kamil, tomek, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13793