Publish Android APK and iOS IPA on GitHub Releases

Don't currently feel like paying for developer accounts to publish it on the respective stores, but can release the builds on GitHub Releases.

APKs should build fine with `expo build:android`, but apparently Expo's iOS build requires an Apple Developer account (due to Fastlane / Xcode API issues). Have to eject and build with Xcode locally to get a standalone IPA, which kinda sucks. See https://github.com/expo/expo/issues/1528

Should also consider publishing to other stores that might be free, e.g. [F-Droid](https://f-droid.org/docs/FAQ_-_App_Developers/?title=FAQ_-_App_Developers)