[web] Introduce Notifications action in thread menu

Summary:
Add action to thread actions menu to launch notifications modal if the user is member of the thread.

Test Plan: Select notifications from thread actions menu to test notifications modal.

Reviewers: palys-swm, benschac, atul

Reviewed By: atul

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3955