[web] [refactor] create modal context, use for setModal

Summary:
adds modal context to be used instead of internal state so we don't have to prop drill

Depends On: D3214

Test Plan: n/a this doesn't do anything yet.

Reviewers: atul, ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3215