Make it possible to set default address book for new contacts

Is your feature request related to a problem? Please describe.
I started with a fresh installation of Nextcloud 19 and am manually populating the address book. I first created 'My contacts' (which is private) and then 'Family contacts' (which is to be editable by my parents). Most of my contacts should go in my personal address book, only a few in the family one. However, each new contact is set in the family address book, and I need to change it every single time - which is frustrating.
As a workaround I now disabled the family address book until I'm adding a contact that should go there. This is equally annoying as (logically) the address book field is hidden when only one address book is activated (and requires often changing the app settings).

Describe the solution you'd like
Have an extra setting in the app's Settings section in the web interface to set the default address book for new contacts. This could either be a dropdown menu, or a line stating the current default with an 'edit' button, which would lead to a modal window to change the setting (similar to the 'import' modal). The setting should be invisible when only one address book is enabled.

Describe alternatives you've considered

• Alternatively a setting that can be changed via the occ command would be great if an option in the web interface would not be possible.
• Set the first created address book as the default (as it may often be assumed the most important one). But this might not work for other users.

The issue names the web interface's Settings section, the import modal as a UI precedent, and an optional occ command; no file or test is identified. Start by tracing how the current address book is selected for new contacts and how app settings are persisted. Done means users can choose a default when multiple address books are enabled, and new contacts use it.