Prepare support for theming

[bodymindarts]

In order to enable multiple apps being build from the same code-base we must extract all bitcoin-beach specific content + themes in a way that allows us to switch them based on static config.

Here are some useful resources to consider:
https://blog.swmansion.com/build-and-publish-multiple-rn-apps-from-the-same-codebase-part-ii-815d0fffc222
https://github.com/fastlane/examples/tree/master/SunApps
https://github.com/fastlane/examples/blob/e82ae4c9ef4102c25855a78f0d22c3a7854150cf/SunApps/Fastfile#L28

(from #107 (comment))

start by defining a json file or directory per app containing assets, colors, features, app store description, screenshots and configuration needed for each app. On CI I would have job per app running in parallel. This allows you to easily see if any app builds and deploys fail. For each job I would define an environment variable e.g. app: bitcoin_beach, that maps to the specific configuration, makes necessary changes to the iOS and Android configurations and then simply runs the Galoy fastfile with the right values substituted.

Expo app config for reference https://docs.expo.io/versions/latest/config/app/