[web] Add button to open pending sidebars

Test Plan: Check if sidebar tooltip displays correct text and navigates to correct thread (empty pending or existing sidebar), check if creating sidebars is possible only when permission is set and message creator is not blocking user

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D749