[web] Use threadUIName instead of robotextName in ThreadSettingsModal

Summary:
I'm about to deprecate robotextName. This diff updates ThreadSettingsModal to figure out a placeholder by resolving a version of the ThreadInfo without the name set.

Depends on D6591

Test Plan:
Open and play around with ThreadSettingsModal, clear name to see placeholder

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6592