[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