(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/client/components/ActionLog.ts

@@ -261,6 +261,7 @@ export class ActionLog extends dispose.Disposable implements IDomComponent {
   private async _loadActionSummaries() {
     if (this._loaded || !this._gristDoc) { return; }
     this._loading(true);
+    // Returned actions are ordered with earliest actions first.
     const result = await this._gristDoc!.docComm.getActionSummaries();
     this._loading(false);
     this._loaded = true;
@@ -268,7 +269,7 @@ export class ActionLog extends dispose.Disposable implements IDomComponent {
     result.forEach(item => this.pushAction(item));
     // Add any actions that came in while we were fetching.  Unlikely, but
     // perhaps possible?
-    const top = result[0] ? result[0].actionNum : 0;
+    const top = result.length > 0 ? result[result.length - 1].actionNum : 0;
     for (const item of this._pending) {
       if (item.actionNum > top) { this.pushAction(item); }
     }