(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/useractions.py

@@ -319,6 +319,14 @@ class UserActions(object):
     """
     pass
 
+  @useraction
+  def UpdateCurrentTime(self):
+    """
+    Somewhat similar to Calculate, trigger calculation
+    of any cells that depend on the current time.
+    """
+    self._engine.update_current_time()
+
   #----------------------------------------
   # User actions on records.
   #----------------------------------------