[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