Fix group editing

[DL6ER]

What does this implement/fix?

Fix editing of groups not being possible. We need to use the group name's for the REST API, not their database ID. We encode the names for safety as users could have used conflicting characters in groups names. This is not new altogether - we do the same for lists as they are allowed to contain / which obviously have some meaning in URLs.

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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[DL6ER]

Context: https://discourse.pi-hole.net/t/activate-deactivate-groups-via-cron-in-v6/67397/6

[yubiuser]

Works fine now, before editing was not possible.

Should we add the name of the group to the notification as we have it for adding/deleting?