[web] Make long thread names end with ellipsis in subchannels modal

Summary:
Issue: https://linear.app/comm/issue/ENG-2638/ellipsis-in-subchannels-modal-on-web
before:

now:

Test Plan: Check the subchannels modals long titles end with ellipsis, check that the style change did not affect its other usage - the message preview.

Reviewers: przemek, michal, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6195