[web] Introduce menu context for displaying actions menu

Summary:
As we need to display menu on the top of UI, I created React context that can handle rendering menu items.
The diff introduces the context that provides function allowing to render items in portal.
Rendering in the portal is not used yet. It will be in the following diffs.

Test Plan: No behavior change in web app. Empty portal div should appear in DOM as a child of react-root div. The portal can be tested after introducing rendering in one of the following diffs.

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3375