[lib] Remove permissions field from ThreadInfo.members[memberID].permissions
Summary:
We're no longer using member permissions anywhere on client (or anywhere ThreadInfo is used which includes eg send.js), so it can be removed.
NOTE: Marked this as DRAFT because I think we can do better than the invariant in memberHasAdminRole, but I think it would require some flow refactoring which I want to defer for now until I get rest of migration stack up.
Depends on D12259
Test Plan: flow
Reviewers: ashoat, ginsu, tomek, varun
Reviewed By: ashoat
Differential Revision: https://phab.comm.dev/D12265