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.yml

@@ -109,3 +109,10 @@
   subject: '12345'
   updated: 2018-12-02 15:13:12
   created: 2018-12-01 15:13:12
+- id: 15
+  username: 'user15'
+  password: '$2a$14$dcadBoMBL9jQoOcZK8Fju.cy0Ptx2oZECkKLnaa8ekRoTFe1w7To.' # 1234
+  email: 'user15@example.com'
+  issuer: local
+  updated: 2018-12-02 15:13:12
+  created: 2018-12-01 15:13:12