[lib] Permissions for thick threads

Summary:

1. Updates getRolePermissionBlobs, which is called from the migration-related function legacyUpdateRolesAndPermissions. It's important that we support thick threads there so migrations don't break on them
2. Introduces getThickThreadRolePermissionsBlob, which is called by the above, and will be called in a later diff to get the permissions for thick threads

Depends on D12656

Test Plan: I haven't tested this stack outside of Flow. I'd like to propose that we land this so the team can iterate. I'll make sure to test all of the functionality once it's integrated into the rest of the codebase.

Reviewers: tomek, marcin, inka

Reviewed By: inka

Differential Revision: https://phab.comm.dev/D12657