[web] Show settings modal from thread menu

Summary: Add action on Settings item in thread menu. It displays modal with settings.

Test Plan: Select thread with permission to change settings (like personal thread) and select Settings 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/D3332