macOS contacts: prefer personal over app generated

[georgehrke]

https://sabre.io/dav/clients/osx-addressbook/

This PR is adding support for ctag:

This client requires impementation of the proprietary '{http://calendarserver.org/ns/}getctag' property. Without it, it will simply not request any vcards.

This PR renames app-generated to z-app-generated:

OS X addressbook can only ever work with 1 address book. Even though the CardDAV standard makes it easy for users to own more than one addressbook, this is not supported.

Unfortunately this means that if a user has more than one addressbook in their account, only the 'first' will show up, and contacts from other addressbooks are completely hidden to the user.

This can cause for a lot of confusion, and there is no obvious solution other than simply enforce a 1 address book limit for users.

tl;dr:
Without this PR, macOS Contacts will only show the 5 contacts from the contacts interaction addressbook, but all the contacts from the personal addressbook will be gone.

[georgehrke]

/backport stable19

[georgehrke]

While the sabre/dav docs say that the one addressbook limitation affect only OS X 10.6 until OS X 10.10, i'm still affected by it on macOS 10.15. So I guess the docs are not up to date.

[rullzer]

Fine by me

[georgehrke]

Test failures unrelated, merging

[georgehrke]

/backport to stable19