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/users_lists.yml

@@ -46,3 +46,9 @@
   right: 2
   updated: 2018-12-02 15:13:12
   created: 2018-12-01 15:13:12
+- id: 9
+  user_id: 15
+  list_id: 26
+  right: 0
+  updated: 2018-12-02 15:13:12
+  created: 2018-12-01 15:13:12