(core) make it easier to enable Azure storage without setting GRIST_DOCS_S3_BUCKET

Summary:
Previously, absence of `GRIST_DOCS_S3_BUCKET` was equated with absence
of external storage, but that is no longer true now that Azure is
available. Azure could be used by setting `GRIST_DOCS_S3_BUCKET`
but the alternative `GRIST_AZURE_CONTAINER` flag is friendlier.

Test Plan:
confirmed manually that Azure can be configured and
used now without `GRIST_DOCS_S3_BUCKET`

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D3448

app/server/mergedServerMain.ts

@@ -34,7 +34,7 @@ interface ServerOptions extends FlexServerOptions {
   logToConsole?: boolean;  // If set, messages logged to console (default: false)
                            //   (but if options are not given at all in call to main,
                            //    logToConsole is set to true)
-  s3?: boolean;            // If set, documents saved to s3 (default is to check environment
+  externalStorage?: boolean; // If set, documents saved to external storage such as s3 (default is to check environment
                            // variables, which get set in various ways in dev/test entry points)
 }
 
@@ -59,7 +59,7 @@ export async function main(port: number, serverTypes: ServerType[],
   }
 
   if (options.logToConsole) { server.addLogging(); }
-  if (options.s3 === false) { server.disableS3(); }
+  if (options.externalStorage === false) { server.disableExternalStorage(); }
   await server.loadConfig();
 
   if (includeDocs) {