(core) Move Notifier to /ext

Summary:
This makes it possible to configure a SendGrid-based Notifier
instance via a JSON configuration file.

Test Plan: Tested manually.

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3432

app/server/lib/idUtils.ts

@@ -40,3 +40,10 @@ export function makeForkIds(options: { userId: number|null, isAnonymous: boolean
 export function getAssignmentId(docWorkerMap: IDocWorkerMap, docId: string): string {
   return docId;
 }
+
+// Get the externalId to use for an AppSumo user.  AppSumo identifies users by
+// an activation email, so we just use that (with an appsumo/ prefix it allow
+// for other families of id in the future).
+export function getExternalIdForAppSumoUser(email: string) {
+  return `appsumo/${email}`;
+}