[lib] Introduce MemberInfoSansPermissions
Summary:
Introduce MemberInfoSansPermissions, stripPermissionsFromMemberInfo, and some unit tests.
The type will eventually replace MemberInfoWithPermissions in RawThreadInfo, but for now it's convenient to use to "surface" flow issues.
Depends on D12296
Test Plan: flow + unit tests
Reviewers: ashoat, ginsu, tomek, varun
Reviewed By: ashoat, tomek
Differential Revision: https://phab.comm.dev/D12315