[web] Show sidebars modal from thread menu

Summary: Add action on Sidebars item in thread menu. It displays modal with sidebars.

Test Plan: Select thread with sidebars inside and select "Sidebars" from thread menu

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3333