(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

app/server/lib/GranularAccess.ts

@@ -99,7 +99,7 @@ const SURPRISING_ACTIONS = new Set([
                                    ]);
 
 // Actions we'll allow unconditionally for now.
-const OK_ACTIONS = new Set(['Calculate']);
+const OK_ACTIONS = new Set(['Calculate', 'UpdateCurrentTime']);
 
 /**
  * Granular access for a single bundle, in different phases.