[lib] Introduce new ifJustViewer field in ThreadEntity for PRIVATE threads

Summary:
We currently name new ENS users' PRIVATE threads after their wallet address. In order to instead display their ENS name, we want to make the EntityText framework support some way to render an unnamed PRIVATE thread's name as just their username.

Currently we display such threads as "just you". In this diff, we introduce a new ifJustViewer field to make it possible to show "ashoat.eth" instead.

Test Plan: In combination with the following diff, I tested this by deleting and recreating my ashoat.eth account in my local web environment, and confirming that my new PRIVATE thread was displayed with the name "ashoat.eth"

Reviewers: tomek, atul

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D7453