(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/common/AsyncCreate.ts

@@ -4,7 +4,7 @@
  * On failure, the result is cleared, so that subsequent calls attempt the creation again.
  *
  * Usage:
- *  this._obj = AsyncCreate<MyObject>(asyncCreateFunc);
+ *  this._obj = new AsyncCreate<MyObject>(asyncCreateFunc);
  *  obj = await this._obj.get();    // calls asyncCreateFunc
  *  obj = await this._obj.get();    // uses cached object if asyncCreateFunc succeeded, else calls it again.
  *

app/common/DocumentSettings.ts

@@ -1,4 +1,11 @@
 export interface DocumentSettings {
   locale: string;
   currency?: string;
+  engine?: EngineCode;
 }
+
+/**
+ * The back-end will for now support at most two engines, a pynbox-backed python2 and
+ * a gvisor-backed python3.
+ */
+export type EngineCode = 'python2' | 'python3';

app/common/gristUrls.ts

@@ -1,5 +1,6 @@
 import {BillingPage, BillingSubPage, BillingTask} from 'app/common/BillingAPI';
 import {OpenDocMode} from 'app/common/DocListAPI';
+import {EngineCode} from 'app/common/DocumentSettings';
 import {encodeQueryParams, isAffirmative} from 'app/common/gutil';
 import {localhostRegex} from 'app/common/LoginState';
 import {LocalPlugin} from 'app/common/plugin';
@@ -426,6 +427,10 @@ export interface GristLoadConfig {
   // If set, enable anonymous sharing UI elements.
   supportAnon?: boolean;
 
+  // If set, allow selection of the specified engines.
+  // TODO: move this list to a separate endpoint.
+  supportEngines?: EngineCode[];
+
   // Max upload allowed for imports (except .grist files), in bytes; 0 or omitted for unlimited.
   maxUploadSizeImport?: number;