[server] Prevent multi-line thread titles

Summary: This diff is the final in the series about multi-line thread titles. It makes sure that the server code trims all thread titles it gets to a single line.

Test Plan: Flow, careful inspection

Reviewers: atul, karol-bisztyga, palys-swm

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D865