Ask active users to leave us a review on the app store

We should prompt active users to review our app on the store.

Based on previous experience, this has a strong positive impact on ratings, as it means that active, happy users are leaving reviews in large numbers, not just people who sought out somewhere to complain. It also has a strong effect on review volume, which tends to mean the app store ranks you higher.

Equivalent Android issue: https://github.com/vector-im/element-android/issues/3876