(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/test_user.py

@@ -0,0 +1,54 @@
+from user import User
+import test_engine
+import testsamples
+
+class TestUser(test_engine.EngineTestCase):
+  # pylint: disable=no-member
+  def setUp(self):
+    super(TestUser, self).setUp()
+    self.load_sample(testsamples.sample_students)
+
+  def test_constructor_sets_user_attributes(self):
+    data = {
+      'Access': 'owners',
+      'Name': 'Foo Bar',
+      'Email': 'email@example.com',
+      'UserID': 1,
+      'LinkKey': {
+        'Param1': 'Param1Value',
+        'Param2': 'Param2Value'
+      },
+      'Origin': 'https://getgrist.com',
+      'StudentInfo': ['Students', 1]
+    }
+    u = User(data, self.engine.tables)
+    self.assertEqual(u.Name, 'Foo Bar')
+    self.assertEqual(u.Email, 'email@example.com')
+    self.assertEqual(u.UserID, 1)
+    self.assertEqual(u.LinkKey.Param1, 'Param1Value')
+    self.assertEqual(u.LinkKey.Param2, 'Param2Value')
+    self.assertEqual(u.Access, 'owners')
+    self.assertEqual(u.Origin, 'https://getgrist.com')
+    self.assertEqual(u.StudentInfo.id, 1)
+    self.assertEqual(u.StudentInfo.firstName, 'Barack')
+    self.assertEqual(u.StudentInfo.lastName, 'Obama')
+    self.assertEqual(u.StudentInfo.schoolName, 'Columbia')
+
+  def test_setting_is_sample_substitutes_attributes_with_samples(self):
+    data = {
+      'Access': 'owners',
+      'Name': None,
+      'Email': 'email@getgrist.com',
+      'UserID': 1,
+      'LinkKey': {
+        'Param1': 'Param1Value',
+        'Param2': 'Param2Value'
+      },
+      'Origin': 'https://getgrist.com',
+      'StudentInfo': ['Students', 1]
+    }
+    u = User(data, self.engine.tables, is_sample=True)
+    self.assertEqual(u.StudentInfo.id, 0)
+    self.assertEqual(u.StudentInfo.firstName, '')
+    self.assertEqual(u.StudentInfo.lastName, '')
+    self.assertEqual(u.StudentInfo.schoolName, '')