[web] initialize localforage on main thread

Summary:
On Safari we need to persist key from main thread. (context)

Depends on D8116

Test Plan:

1. Try to add data to indexedDB from main thread
2. Check in database on worker still works

Reviewers: michal, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8117