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/tasks.yml

@@ -355,3 +355,17 @@
   created: 2018-12-01 01:12:04
   updated: 2018-12-01 01:12:04
   due_date: 2018-10-30 22:25:24
+- id: 39
+  uid: 'uid-caldav-test'
+  title: 'Title Caldav Test'
+  description: 'Description Caldav Test'
+  priority: 3
+  done: false
+  created_by_id: 15
+  list_id: 26
+  index: 39
+  due_date: 2023-03-01 15:00:00
+  created: 2018-12-01 01:12:04
+  updated: 2018-12-01 01:12:04
+  bucket_id: 1
+  position: 39