[web] Introduce common Menu component

Summary:
Excluded threadMenu logc into separate Menu component. Changes names from ThreadMenu... to Menu....
The goal is to use the component in different places where menu with actions is needed.

Test Plan: Run web app, thread actions menu in top right corner should still look and behave in the same way.

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, atul, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3374