Allow group plans for private guilds only (#12479)

* hide upgrade button for public guilds

* prevent group plans for public guilds

* fix unit tests

* fix integration and unit tests that assumed group plans could be public guilds

* more unit tests fixes

* more resilient unit test

* more resilient unit test

test/api/unit/libs/payments/amazon/upgrade-groupplan.test.js

@@ -31,11 +31,14 @@ describe('#upgradeGroupPlan', () => {
     group = generateGroup({
       name: 'test group',
       type: 'guild',
-      privacy: 'public',
+      privacy: 'private',
       leader: user._id,
     });
     await group.save();
 
+    user.guilds.push(group._id);
+    await user.save();
+
     spy = sinon.stub(amzLib, 'authorizeOnBillingAgreement');
     spy.resolves([]);