[feature] Frontend - Group Selection in Mass Upgrade UI

[youhaveme9]

Title

Add Group Selection UI for Mass Firmware Upgrades

Context / Approach

To support group-based mass firmware upgrades, we need to update the admin interface to allow selecting device groups when initiating a mass upgrade.

My approach will be to:

1. Add a group autocomplete field in the BatchUpgradeConfirmationForm class
2. Add a group filter in the BatchUpgradeOperationAdmin list view
3. Update the admin templates to display group information in the upgrade confirmation and detail views
4. Update API endpoints and serializers to support group-based filtering
5. Update the README documentation to include information about group-based upgrades in both the intro and mass upgrade explanation sections

The autocomplete field will improve performance when dealing with a large number of groups, and the filter will make it easier to find past operations related to specific groups.

The UI implementation will follow OpenWISP's design patterns and maintain consistency with other modules that use group selection.

Dependencies

• Depends on the backend implementation of group-based mass firmware upgrades