trim outside of getMessageTitle

Summary: Trim message title only for the sidebar title, omit message previews

Test Plan: create sidebars from long messages, see the preview of the long messages and the sidebars - sidebars titles should be trimmed and message previews shouldn't

Reviewers: palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D794