feat(caldav): import caldav categories as Labels (#1413)

Resolves #1274

Co-authored-by: ce72 <christoph.ernst72@googlemail.com>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1413
Reviewed-by: konrad <k@knt.li>
Co-authored-by: cernst <ce72@noreply.kolaente.de>
Co-committed-by: cernst <ce72@noreply.kolaente.de>

pkg/db/fixtures/lists.yml

@@ -253,3 +253,13 @@
   position: 8
   updated: 2018-12-02 15:13:12
   created: 2018-12-01 15:13:12
+-
+  id: 26
+  title: List 26 for Caldav tests
+  description: Lorem Ipsum
+  identifier: test26
+  owner_id: 15
+  namespace_id: 18
+  position: 1
+  updated: 2018-12-02 15:13:12
+  created: 2018-12-01 15:13:12