[lib] Introduce useENSNames to fetch more than one ENS name at a time

Summary: Also rewrite useStringForUser to call into useENSNames.

Test Plan: Tested existing calls of useStringForUser, both when logged in as an ENS user and when not

Reviewers: atul, tomek

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6423