[web] Introduce Leave thread menu action

Summary:
Added Leave Thread option.
It's functionality after click will be added in separate diff.

Test Plan: Tested if the item is displayed in threads, that can be left. It shouldn't appear in Personal threads.

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3192