[web] Find sidebarParent in threadList and make sure active sidebar is included in sidebars

Summary: Follow-up to D956: instead of iterating over threadListData, use findIndex to get position of sidebar parent, make sure active sidebar is included and change parent in array

Test Plan: Flow, check if active sidebar is included

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1036