Add save-to-cloud feature #8
Description
It should be possible to save the user data (the state of which markers are visited/bookmarked, plus other persistent settings) to the cloud so that the user can restore this data if they reinstall the app or switch to a different mobile device. This should also be integrated under the settings page (#6).
Point for consideration: should this be a platform-agnostic feature or should this be customized between Android vs. iOS? The advantage of a platform-agnostic feature is that users can store data even if they switch mobile platforms.