[lib] Update messageContentFor[Client/Server]DB to handle UpdateRelationship

Summary:
Based on https://github.com/CommE2E/comm/commit/c46628b1139850005d11f057298d18896b1af30d and https://github.com/CommE2E/comm/commit/06cb5b0bd357b1d02aaf858eb0cd57ab5996cde9 from hackathon branch

Depends on D11439

Test Plan: This should be a noop, flow. Checked MariaDB messages table after subsequent diffs to make sure that neynar_mutual appeared as expected: https://blob.sh/9c626e.png

Reviewers: ginsu, tomek, will, varun

Reviewed By: ginsu

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11443