fix(list): when list background is removed, delete file from file system and DB (#1372)

Co-authored-by: testinho.testador <testinho.testador@noreply.kolaente.de>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1372
Reviewed-by: konrad <k@knt.li>
Co-authored-by: clos <clos@noreply.kolaente.de>
Co-committed-by: clos <clos@noreply.kolaente.de>

pkg/db/fixtures/lists.yml

@@ -242,3 +242,14 @@
   position: 7
   updated: 2018-12-02 15:13:12
   created: 2018-12-01 15:13:12
+-
+  id: 25
+  title: Test25 with background
+  description: Lorem Ipsum
+  identifier: test6
+  owner_id: 6
+  namespace_id: 6
+  background_file_id: 1
+  position: 8
+  updated: 2018-12-02 15:13:12
+  created: 2018-12-01 15:13:12