(core) Billing updates

Summary:
- Adding confirmation dialog when user doesn't want to cancel site
- Changing `Cancel subscription` to `Cancel plan`
- Removing `Pro` from upgrade header on pricing modal
- Better handling situation when there is no default price
- Removing mentions about sprouts program
- Removing cache for stripe plans

Test Plan: Updated tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D4273

test/nbrowser/HomeIntro.ts

@@ -87,7 +87,7 @@ describe('HomeIntro', function() {
 
       // Check message specific to logged-in user and an empty team site.
       assert.match(await driver.find('.test-welcome-title').getText(), new RegExp(`Welcome.* ${session.orgName}`));
-      assert.match(await driver.find('.test-welcome-text').getText(), /Learn more.*find an expert/);
+      assert.match(await driver.find('.test-welcome-text').getText(), /Learn more/);
       assert.notMatch(await driver.find('.test-welcome-text').getText(), /sign up/);
     });