(core) support setting python version of new docs with PYTHON_VERSION_ON_CREATION

Summary:
If PYTHON_VERSION_ON_CREATION is set in the environment, new documents will be created with a specific desired python version (2 or 3).

This diff commits to offering a choice of engine, so the engine for a document no longer starts to initialize until the document has been fetched and read. Staging (and dev, and testing) has been like this for a while.

Test Plan: added test; manual testing of forks/copies etc

Reviewers: dsagal, alexmojaki

Reviewed By: dsagal, alexmojaki

Differential Revision: https://phab.getgrist.com/D3119

app/server/lib/serverUtils.ts

@@ -161,10 +161,3 @@ export function getSupportedEngineChoices(): EngineCode[]|undefined {
   }
   return undefined;
 }
-
-/**
- * Check whether a choice of engine is supported.
- */
-export function supportsEngineChoices(): boolean {
-  return getSupportedEngineChoices() !== undefined;
-}