[web] use context to display Menu action items

Summary:
Introduce rendering menu items using function provided by menu context. Thread actions menu should now have correct position.
As we need to set correct position of the content in portal, I used getBoundingClientRect react function, which calculates position of rendered element.
The position needs to be updated window resizes.

Test Plan: In web app, click on thread actions menu. It should appear in correct place and in browser's inspector it will be in the portal.

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, benschac, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3377