Add additional paywall events

Please, add additional monetization events so we can analyze the funnel in more depth:

1. Paywall views (total)

2. Paywall button clicks (unique)
3. Checkout views (unique)

4. Paywall views (unique) → Paywall button clicks (unique) CR

5. Paywall button clicks (unique) → Checkout views (unique) CR

6. Checkout views (unique) → Purchases CR