[web] fix leave channel modal for long thread names

Summary:
This fixes a bug where the modal would gorw really wide if the thread name has a really long name. This diff fixes this bug

Linear task: https://linear.app/comm/issue/ENG-8438/leave-channel-modal-on-web-grows-when-a-thread-has-a-really-long-name

Test Plan:
Please see the screenshots

Before:

After:

Reviewers: inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12582