[server] Update roles before adding new members in updateThread

Summary: If we call changeRole for the new members before updating the permissions for their role, they will end up with the wrong permissions.

Test Plan: I'm going to submit a whole bunch of permissions diffs and will aim to test them together later. This one should be pretty clear, though

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1006