Design the onboarding user experience

Currently, we decide at build time the default backend for Cristal users.

For the web version, this can be defined by the administrator.

For the electron version, the default values are hardcoded and can be changed by editing a json configuration file manually.

This process has a lot of room for improvements. We should aim for a scenario where the user starts from an empty configuration, and gradually configure their instance with their own configuration.

Interesting points:

• choice of backend type and backend endpoint (e.g., the url of a given Nextcloud or XWiki instance)
• choice of design system
• choice of skin
• other user preferences

Note that based on where and how the instance is deployed, some part of the configuration could be disabled

e.g., if Cristal is embeded in a Nextcloud instance, it shouldn't be possible for users to add new backends, switch design system and so on, while users of their own electron instance should be pretty free of their choices (though we might want to have a advanced mode with more options)