[web] Introduce web migration to strip member permissions

Summary:
Effectively the same as D12789, but for web.

Depends on D12789

Test Plan:
Before (thin, with member permissions):

Before (thick, with member permissions):

After (thin, WITHOUT member permissions):

After (thick, still WITH member permissions):

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D12790