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/namespaces.yml

@@ -88,3 +88,9 @@
   owner_id: 12
   updated: 2018-12-02 15:13:12
   created: 2018-12-01 15:13:12
+- id: 18
+  title: testnamespace18
+  description: Lorem Ipsum
+  owner_id: 15
+  updated: 2018-12-02 15:13:12
+  created: 2018-12-01 15:13:12