(core) mark engine setting as experimental

Summary:
This marks the engine setting in document settings as experimental,
with a skull and cross-bones.

It also makes sure the setting is shown if PYTHON_VERSION_ON_CREATION
is set (this relates to a separate change to set the default python
version to 3).

Test Plan: manual

Reviewers: alexmojaki, dsagal

Reviewed By: alexmojaki, dsagal

Subscribers: anaisconce

Differential Revision: https://phab.getgrist.com/D3120

app/server/lib/serverUtils.ts

@@ -155,7 +155,8 @@ export function getLogMetaFromDocSession(docSession: OptDocSession) {
  * Only offer choices of engine on experimental deployments (staging/dev).
  */
 export function getSupportedEngineChoices(): EngineCode[]|undefined {
-  if (process.env.GRIST_EXPERIMENTAL_PLUGINS === '1') {
+  if (process.env.GRIST_EXPERIMENTAL_PLUGINS === '1' ||
+      process.env.PYTHON_VERSION_ON_CREATION) {
     return ['python2', 'python3'];
   }
   return undefined;