Stable identifier for users

[kthchew]

Problem To Be Solved

Currently the backend only identifies users by their email

Motivation

Users may change their email (there is currently no functionality to do this, but this is a fairly common action) and we should still have a stable ID that will not change to identify them

Proposed Solution

Store a randomly generated UUID in the database for each user (could also use a username, but unless we don't allow username changes that identifier still won't be stable)