(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/common/gutil.ts

@@ -899,9 +899,9 @@ export function isAffirmative(parameter: any): boolean {
  * Returns whether a value is neither null nor undefined, with a type guard for the return type.
  *
  * This is particularly useful for filtering, e.g. if `array` includes values of type
- * T|null|undefined, then TypeScript can tell that `array.filter(isObject)` has the type T[].
+ * T|null|undefined, then TypeScript can tell that `array.filter(isNonNullish)` has the type T[].
  */
-export function isObject<T>(value: T | null | undefined): value is T {
+export function isNonNullish<T>(value: T | null | undefined): value is T {
   return value !== null && value !== undefined;
 }