(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/functions/date.py

@@ -447,10 +447,11 @@ def NOW(tz=None):
   """
   Returns the `datetime` object for the current time.
   """
+  engine = docmodel.global_docmodel._engine
+  engine.use_current_time()
   return datetime.datetime.now(_get_tzinfo(tz))
 
 
-
 def SECOND(time):
   """
   Returns the seconds of `datetime`, as an integer from 0 to 59.