Remove e-mail usage #63
Description
Per #55, e-mail is not a consistent identifier across Zulip and RC API. Or across Zulip in time.
After #62, the only remaining use of e-mail identifiers is in the "reviews" feature. We could replace this with Zulip IDs to make the migration away from e-mail complete.
(Note: we don't want to store names here; that might lead the review to present a name that is no longer appropriate for the reviewer.)