fix(contactsinteraction): Allow vCard download

Sabre calculates a header for the size of a vcard, therefore we have to
implement the size method.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

apps/contactsinteraction/lib/Card.php

@@ -96,7 +96,7 @@ public function getETag(): ?string {
 	 * @inheritDoc
 	 */
 	public function getSize(): int {
-		throw new NotImplemented();
+		return strlen($this->contact->getCard());
 	}
 
 	/**