[web] Convert Modal into functional component

Summary: Convert exiting Modal component from class into functional component to simplify the logic and make future changes easier. No logic changes has been made.

Test Plan: All existing modals should work as before

Reviewers: palys-swm, atul, benschac

Reviewed By: atul

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

Differential Revision: https://phabricator.ashoat.com/D3667