Summary:
The 'user' variable has a similar API to the one from access rules: it
contains properties about a user, such as their full name and email
address, as well as optional, user-defined attributes that are populated
via user attribute tables.
Test Plan: Python unit tests.
Reviewers: alexmojaki, paulfitz, dsagal
Reviewed By: alexmojaki, dsagal
Subscribers: paulfitz, dsagal, alexmojaki
Differential Revision: https://phab.getgrist.com/D2898
