Launch-time tasks

[gnprice]

This is an issue for recording things we'll want to do around the time of launching the app to GA (replacing the legacy zulip-mobile app), and that we'd otherwise be at risk of forgetting.

• Switch app icons to the non-"beta" versions (see logo: Use new design, and support fancy Android features #1404)
• Update the names seen in /stats (currently "Mobile app (React Native)" and "Mobile app beta (Flutter)")
• Update the Help Center for changes to the app (try to think of things; also look through open post-launch issues for things that are features in the legacy app; also grep through the Help Center source for "mobile" tabs)
• Reset ratings and reviews in the App Store and Play Store
• (There'll be lots of other things to do too; we'll fill out a list of them more fully when the launch gets closer. For now this is only to track things we think of now and feel we should write down in order not to forget.)