(core) Adding new column in users table "ref" with unique identifier.

Summary:
There is a new column in users table called ref (user reference).
It holds user's unique reference number that can be used for features
that require some kind of ownership logic (like comments).

Test Plan: Updated tests

Reviewers: georgegevoian, paulfitz

Reviewed By: georgegevoian, paulfitz

Differential Revision: https://phab.getgrist.com/D3641

sandbox/grist/test_completion.py

@@ -10,6 +10,7 @@ class TestCompletion(test_engine.EngineTestCase):
   user = {
     'Name': 'Foo',
     'UserID': 1,
+    'UserRef': '1',
     'StudentInfo': ['Students', 1],
     'LinkKey': {},
     'Origin': None,
@@ -103,6 +104,7 @@ class TestCompletion(test_engine.EngineTestCase):
         ('user.SessionID', "'u1'"),
         ('user.StudentInfo', 'Students[1]'),
         ('user.UserID', '1'),
+        ('user.UserRef', "'1'"),
       ]
     )
     # Should follow user attribute references and autocomplete those types.
@@ -130,6 +132,7 @@ class TestCompletion(test_engine.EngineTestCase):
       'Email': 'baro@example.com',
       'LinkKey': {},
       'UserID': 2,
+      'UserRef': '2',
       'Access': 'owners',
       'SessionID': 'u2',
       'IsLoggedIn': True
@@ -145,6 +148,7 @@ class TestCompletion(test_engine.EngineTestCase):
         ('user.Origin', 'None'),
         ('user.SessionID', "'u2'"),
         ('user.UserID', '2'),
+        ('user.UserRef', "'2'"),
       ]
     )
     self.assertEqual(