[web] Fix issue with fetching messages after creating pending thread

Summary:
Fixes the issue: https://linear.app/comm/issue/ENG-1170/after-creating-any-pending-threadsidebar-on-web-app-dont-fetch-any
There are two changes introduced in the diff:

• remove pendingThread from navInfo in nav-reducer if activeChatThreadID is not pending
• replace checking pendingThread with checking if current threadInfo is pending in chat-message-list.react.js

Each of these things should fix the original issue, but I think both of them should be introduced to prevent accidentally re-introducing the issue in the future

Test Plan: Follow steps described in Linear task. Messages should be always fetched from server

Reviewers: palys-swm, atul, benschac

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4095