Handle foreign address books

[call-me-matt]

Make sure a background job has access to the different user's address books.

[call-me-matt]

Implemented and working, by creating a ContactsManager. There is a promising interface in Nextcloud v19, though: the IAddressBookProvider. Unfortunately I could not figure out how to use it until now, see help request.
The implementation should be optimized by

• ignoring the system address book
• ignoring disabled address books
• operating only on user owned address books (not the shared ones from other users)

[call-me-matt]

System address books and shared address books can be filtered from NCv20 on:
nextcloud/server#22098
Deactivated address books seem to be more complicated:
nextcloud/server#17537