[Feature Request] import/export system rules

### Summary:
An import/export feature for firewall rules just as it works with application rules.

As of now I just copy /etc/opensnitchd/system-fw.json but it would be great if it could be done from the gui as-well.

Start by tracing how the GUI imports and exports application rules, then compare that flow with /etc/opensnitchd/system-fw.json. The feature is complete when system firewall rules can be imported and exported through the GUI, matching the existing application-rule behavior.

Written by the indexing model from the issue text.