Improved performance in Spring v0.13.

We've moved most of the heavy operations into a separate thread  (sync with relays, etc), which means UI is much smoother now. Start up time also got much lower.

There are some other performance optimizations we'll be working on next week, so the UX should get even more pleasant.

Also, there is now an automated build workflow set up, which "will ensure the build process will be transparent and more reproducible to other users/developers". Big thanks to @Rif'at Ahdi R for your contribution!

Play Store: https://play.google.com/store/apps/details?id=com.nostr.universe
APK: https://github.com/nostrband/nostr-universe/releases/download/v0.13.0/spring-nostr-browser-v0.13.0.apk 

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