[web] [refactor] currentModal --> modal

Summary:
Part of moving modal state to a context and hook. There is only ever one modal shown at a time.

The rest of the currentModal props will be removed in upcoming diffs when the new hook is applied/used.

Test Plan: Make sure modals still work

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3194