[feature] Add backup and restore functionality for all user data

Is your feature request related to a problem? Please describe.
There is currently no native backup/export and restore/import functionality for all user data, only export and import for devices. This can lead to loss of important data that the user has spent a lot of time and effort configuring.

Describe the solution you'd like
Ideally there should be automatic backups with configurable destination, schedule, retention policy, etc. At the bare minimum an export and an import button for all data (templates, device groups, organizations, ..., not just devices). Backups from the latest release should work on edge and vice-versa.

Describe alternatives you've considered
At the moment the only alternative is to backup/restore the Docker volumes directly (all of them, since users don't know where the data they care about is). This can be cumbersome, unreliable, requires stopping OpenWISP and I suspect it breaks things when switching from latest to edge or vice-versa.

Additional context

No files, tests, or entry points are identified in the issue. Start by mapping how OpenWISP currently exports and imports devices, then determine how templates, device groups, organizations, and other user data are represented. Done would require an agreed scope for export/import or scheduled backups, configurable destinations and retention, and compatibility between latest and edge releases.