Store version in some way on AddressBook

[timbl]

It would be good practice and useful for migrations to record the version number of the data model of the address book (or group).

That could directly or indirectly give a shape against which the data could be validated. We have a SHACL file somewhere.

If we have a shape the data aspires to then we could run the check every now and again, or on user request.

At the moment we have example that we are upgrading the way group lists are stored, and we need too think about how and when to offer migration to the user.

Maybe it should be at the group level not the address book level as things like meetings and chats may have groups which are not in an address book.