(core) fix invite links and add tests with APP_HOME_URL set

Summary:
Invite links broke when some base domain plumbing changed.
This fix updates them to be aware of the base domain,
and tests the Notifier class with APP_HOME_URL set to
make sure the environment variable has the expected effect.

Test Plan: added test, updated tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2630

stubs/app/server/lib/create.ts

@@ -19,7 +19,7 @@ export const create: ICreate = {
       addWebhooks(app: any) { /* do nothing */ }
     };
   },
-  Notifier(dbManager: HomeDBManager, homeUrl: string) {
+  Notifier() {
     return {
       get testPending() { return false; }
     };