[web] Add pending sidebar to chat thread list as active item

Summary: When creating sidebars on web, we want to add them to chat thread list as active, below parent thread.

Test Plan: Check if opening pending sidebar results in thread added to chat thread list with correct data

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, zrebcu411, Adrian, atul, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D795