(core) Update the current time in formulas automatically every hour

Summary: Adds a special user action `UpdateCurrentTime` which invalidates an internal engine dependency node that doesn't belong to any table but is 'used' by the `NOW()` function. Applies the action automatically every hour.

Test Plan: Added a Python test for the user action. Tested the interval periodically applying the action manually: {F43312}

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3389

sandbox/grist/table.py

@@ -60,8 +60,6 @@ class UserTable(object):
   def __init__(self, model_class):
     docmodel.enhance_model(model_class)
     self.Model = model_class
-    column_ids = {col for col in model_class.__dict__ if not col.startswith("_")}
-    column_ids.add('id')
     self.table = None
 
   def _set_table_impl(self, table_impl):