(core) Store formula values in DB, and include them into .stored/.undo fields of actions.

Summary:
- Introduce a new SQLiteDB migration, which adds DB columns for formula columns
- Newly added columns have the special ['P'] (pending) value in them
  (in order to show the usual "Loading..." on the first load that triggers the migration)
- Calculated values are added to .stored/.undo fields of user actions.
- Various changes made in the sandbox to include .stored/.undo in the right order.
- OnDemand tables ignore stored formula columns, replacing them with special SQL as before
- In particular, converting to OnDemand table leaves stale values in those
  columns, we should maybe clean those out.

Some tweaks on the side:
- Allow overriding chai assertion truncateThreshold with CHAI_TRUNCATE_THRESHOLD
- Rebuild python automatically in watch mode

Test Plan: Fixed various tests, updated some fixtures. Many python tests that check actions needed adjustments because actions moved from .stored to .undo. Some checks added to catch situations previously only caught in browser tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2645

app/server/lib/Sharing.ts

@@ -213,9 +213,10 @@ export class Sharing {
     // action that initializes them when the document is opened cold
     // (without cached ActiveDoc) - otherwise we'll end up with spam
     // log entries for each time the document is opened cold.
-    const trivial = (userActions.length === 1 &&
-                     userActions[0][0] === 'Calculate' &&
-                     sandboxActionBundle.stored.length === 0);
+
+    const isCalculate = (userActions.length === 1 &&
+                         userActions[0][0] === 'Calculate');
+    const trivial = isCalculate && sandboxActionBundle.stored.length === 0;
 
     const actionNum = trivial ? 0 :
       (branch === Branch.Shared ? this._actionHistory.getNextHubActionNum() :
@@ -256,7 +257,9 @@ export class Sharing {
           // be shared. Once sharing is enabled, we would share a snapshot at that time.
           await this._actionHistory.recordNextShared(localActionBundle);
         }
-        if (client && client.clientId) {
+        // Check isCalculate because that's not an action we should allow undo/redo for (it's not
+        // considered as performed by a particular client).
+        if (client && client.clientId && !isCalculate) {
           this._actionHistory.setActionClientId(localActionBundle.actionHash!, client.clientId);
         }
       });
@@ -274,6 +277,9 @@ export class Sharing {
       client,
       retValues: sandboxActionBundle.retValues,
       summarize: true,
+      // Mark the on-open Calculate action as internal. In future, synchronizing fields to today's
+      // date and other changes from external values may count as internal.
+      internal: isCalculate,
     });
     await this._activeDoc.broadcastDocUpdate(client || null, 'docUserAction', {
       actionGroup,