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/buckets.yml

@@ -218,3 +218,9 @@
   created_by_id: -2
   created: 2020-04-18 21:13:52
   updated: 2020-04-18 21:13:52
+- id: 36
+  title: testbucket36
+  list_id: 26
+  created_by_id: 15
+  created: 2020-04-18 21:13:52
+  updated: 2020-04-18 21:13:52