(core) add a python3 button

Summary: This adds a dropdown to the document settings model in staging/dev to set the python engine to Python2 or Python3. The setting is saved in `_grist_DocInfo.documentSettings.engine`.

Test Plan: tested manually for now - separate diff needed to add runsc to jenkins setup and make this testable

Reviewers: dsagal, alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3014

app/server/lib/Sharing.ts

@@ -246,6 +246,16 @@ export class Sharing {
       // when we come to it. For now we only log skipped envelopes as "alien" in _logActionBundle().
       const ownActionBundle: LocalActionBundle = this._filterOwnActions(localActionBundle);
 
+      // If the document has shut down in the meantime, and this was just a "Calculate" action,
+      // return a trivial result.  This is just to reduce noisy warnings in migration tests.
+      if (this._activeDoc.isShuttingDown && isCalculate) {
+        return {
+          actionNum: localActionBundle.actionNum,
+          retValues: [],
+          isModification: false
+        };
+      }
+
       // Apply the action to the database, and record in the action log.
       if (!trivial) {
         await this._activeDoc.docStorage.execTransaction(async () => {