[identity] don't let linkFarcasterAccount overwrite existing fid for user

Summary:
https://linear.app/comm/issue/ENG-7358/make-sure-only-1-fid-is-associated-with-a-given-comm-user

Depends on D11410

Test Plan: tried calling linkFarcasterAccount multiple times. attempts with the same FID succeeded but attempts with a different FID failed.

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11411