Backup/Restore over the admin panel

[sven-n]

Is your feature request related to a problem? Please describe.
For backup and debugging purposes it would be nice to have a simple way to backup and restore the configuration and selected accounts.

Describe the solution you'd like
The admin panel could get a Export and Import functionality on the Setup page of the admin panel. As data format, we could use json, since we have a stable serializer for that which supports circular references and other nasty stuff.

See also JsonDownloadController.

[sd3420625]

This optimization plan is really good. Now I back up and restore directly through the postgres database. As a result, every time I restore, the items sold by the merchant are overwritten and additional deletions are required.
In addition, a small bug was discovered during the operation. After resetting the server data through the admin panel, the data template was not the latest version, causing all subsequent repairs to basically not take effect.