(core) Add 'user' variable to trigger formulas

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

sandbox/grist/usercode.py

@@ -53,7 +53,7 @@ class Address:
   city = grist.Text()
   state = grist.Text()
 
-  def _default_country(rec, table, value):
+  def _default_country(rec, table, value, user):
     return 'US'
   country = grist.Text()