Use/support user ids in preference to emails where possible

[neiljp]

Historically the server/API used only emails as identifiers. This is no longer the case, and rather ids are preferred, for many reasons.

User emails are generated on a per-org basis, so use of emails should still work (?), but we should probably move towards ids.

See https://chat.zulip.org/#narrow/stream/127-integrations/topic/emails.20in.20API/near/834961

[zulipbot]

Hello @zulip/server-refactoring members, this issue was labeled with the "area: refactoring" label, so you may want to check it out!