[lib] Rename action type to set aux user fids

Summary: A simple renaming of the action type. This clarifies that we are setting fids for all comm users not just friends or mutuals.

Test Plan: flow check

Reviewers: ginsu, kamil, varun

Reviewed By: ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11859