'Link' Groups and Address Books

Is your feature request related to a problem? Please describe.

I love the contacts app, but 'Groups' and 'Address books' make little sense the way they are.

Consider this:

1. On the web interface, I cannot 'click' on an address book and see its content or manipulate it. I can only do that via a device. With groups, it's the opposite! I can select a group and look at its content and to an extent manipulate it, but this is irrelevant anywhere else except on the web interface.

2. Even assuming I manually do the job of replicating the content of groups into address books, or vice-versa, this requires constant maintenance. If I modify an address books, by adding a contact to it via a device, I need to manually repeat the operation in regards to the group. Either one or the other are destined to carry outdated data.

3. If I reinstall Nextcloud, I can easily import address books, and have my contacts conveniently organized for devices. Conversely, while I can download groups, I can only re-import them later as address books (?). On a new installation the user has to recreate all the groups once more.

Describe the solution you'd like

I suggest something like this:

Groups could be 'linked' or 'not linked' to Address Books.

1. When importing an address books into the contacts, or selecting it in settings, the user should have the option to 'create a linked group of the same name'.
2. If a user realizes they need their groups to be address books, so that an external device can recognize them, there should be a function to 'create a linked address book from group'.
3. 'Linked' groups and Address books would be automatically kept in sync. If a contact changes Address book, this will apply to the groups also. The user would make their changes on the web interface by looking at groups, knowing the changes are carried through to their linked Address books. No need to repeat a manual action twice!
4. By default, groups would be unlinked from Address books, the way it is now. So an update carrying this functionality wouldn't create any havoc.

No files, tests, or entry points are named. Start by reviewing the existing web-interface handling of groups and address books, then trace how device changes and imports are represented; done means linked and unlinked collections behave as described and remain synchronized without changing the default behavior.