(core) Refactor Table.Record[Set] classes

Summary:
Dealing with some things that bothered and sometimes confused me:

Make Table.Record[Set] provide the table argument automatically
Remove the classes from UserTable because they're not used anywhere and the Table/UserTable distinction is already confusing. They're not documented for users and they don't show up in autocomplete.
Remove RecordSet.Record because it was confusing me where that attribute was being set, but also this means .Record will work properly for users with columns named 'Record'.

Test Plan: existing tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2913

sandbox/grist/test_completion.py

@@ -213,6 +213,8 @@ class TestCompletion(test_engine.EngineTestCase):
   def test_suggest_lookup_methods(self):
     # Should suggest lookup formulas for tables.
     self.assertEqual(self.engine.autocomplete("Address.", "Students", "firstName", self.user), [
+      ('Address.Record', '', True),
+      ('Address.RecordSet', '', True),
       'Address.all',
       ('Address.lookupOne', '(colName=<value>, ...)', True),
       ('Address.lookupRecords', '(colName=<value>, ...)', True),