[web] Show "leave thread" modal from thread menu

Summary: Add action on "Leave thread" item in thread menu. It displays modal with confirmation before leaving thread.

Test Plan: Select thread, that can be left (like some community thread) and select "Leave thread" from thread menu.

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3334