(core) Polishing upgrade plan UI

Summary:
- Update nudge boxes content and collapsing on personal and free team site
- New confirmation after upgrading from a free team site
- Refactoring ProductUpgrade code, splitting plans / modals and nudges

Test Plan: Manual and updated tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3481

app/client/lib/localStorageObs.ts

@@ -1,3 +1,4 @@
+import {safeJsonParse} from 'app/common/gutil';
 import {Observable} from 'grainjs';
 
 /**
@@ -96,3 +97,14 @@ export function localStorageObs(key: string, defaultValue?: string): Observable<
   obs.addListener((val) => (val === null) ? store.removeItem(key) : store.setItem(key, val));
   return obs;
 }
+
+/**
+ * Helper to create a JSON observable whose state is stored in localStorage.
+ */
+ export function localStorageJsonObs<T>(key: string, defaultValue: T): Observable<T> {
+  const store = getStorage();
+  const currentValue = safeJsonParse(store.getItem(key) || '', defaultValue ?? null);
+  const obs = Observable.create<T>(null, currentValue);
+  obs.addListener((val) => (val === null) ? store.removeItem(key) : store.setItem(key, JSON.stringify(val ?? null)));
+  return obs;
+}