[web] threadListSideBar -> threadListSidebar

Summary: Super minor rename addressing @palys-swm feedback: https://phabricator.ashoat.com/D3361?id=10124#inline-19750

Test Plan: Searched repo for threadListSide[B/b]ar and found no other occurrences

Reviewers: palys-swm, def-au1t, varun, benschac, jimpo

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, palys-swm

Differential Revision: https://phabricator.ashoat.com/D3389