[web] Use threadInfo.uiName for child chat modals

Summary: We should always use uiName instead of name, since name can be empty. We do the same thing on native.

Test Plan: I tested this in my local environment and confirmed the empty names became non-empty

Reviewers: jacek, atul

Reviewed By: atul

Subscribers: tomek, adrian, abosh

Differential Revision: https://phab.comm.dev/D4810