fix(runtime): force `react-native-screens` outside snackager

fix(runtime): enable both classic and eas update urls

refactor(snack-sdk): make `channel` generation URL-safe (#528)

* refactor(snack-sdk): make `channel` generation URL-safe

See: https://stackoverflow.com/a/7111947

* refactor(snack-sdk): set a max-length on the channel id

@mike/secrets (#521)

* refactor(snackager): use ExternalSecret

* refactor(snackpub): use ExternalSecret

* refactor(website): remove secret

* refactor(ci): remove git-crypt usage