carddav: support address book creation/deletion

Now that the handling for multiple address books is in place, this
commit adds initial support for creation and deletion of address books.

These operations obviously require support from the backend, so the
interface gains two new methods. All properties of the address book
passed to `CreateAddressBook()` may be unset (e.g. when a client sends a
MKCOL request without a body), except for the path, which is always set.
It is up to the backend to put any desired default values in place.

carddav/carddav_test.go

@@ -68,6 +68,14 @@ func (b *testBackend) GetAddressBook(ctx context.Context, path string) (*Address
 	return nil, webdav.NewHTTPError(404, fmt.Errorf("Not found"))
 }
 
+func (*testBackend) CreateAddressBook(ctx context.Context, ab AddressBook) error {
+	panic("TODO: implement")
+}
+
+func (*testBackend) DeleteAddressBook(ctx context.Context, path string) error {
+	panic("TODO: implement")
+}
+
 func (*testBackend) GetAddressObject(ctx context.Context, path string, req *AddressDataRequest) (*AddressObject, error) {
 	if path == alicePath {
 		card, err := vcard.NewDecoder(strings.NewReader(aliceData)).Decode()