(core) Allowing installation admins to manage billing.

Summary:
Permissions for admin billing endpoints were changed

- Support user can't use admin subscription endpoints
- Installation admin (as support user) can see billing details on any site
- Installation admin (unlike support user) can replace subscription (or attach payment) on any site, regardless permissions

Installation admin is any user that belongs to a special `admin` org. If `admin` org is not defined, it defaults to
support user. In that case, with this diff, the support user receives admin's permissions, and now can replace subscription on
any site (without being billing manager).

Test Plan: Added new test

Reviewers: dsagal, paulfitz

Reviewed By: dsagal, paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D4338

test/nbrowser/WebhookOverflow.ts

@@ -117,7 +117,9 @@ async function openWebhookPageWithoutWaitForServer() {
 async function waitForWebhookPage() {
   await driver.findContentWait('button', /Clear Queue/, 3000);
   // No section, so no easy utility for setting focus. Click on a random cell.
-  await gu.getDetailCell({col: 'Webhook Id', rowNum: 1}).click();
+  await gu.waitToPass(async () => {
+    await gu.getDetailCell({col: 'Webhook Id', rowNum: 1}).click();
+  });
 }
 
 export async function openAccountMenu() {