Make sure list / task favorites are set per user, not per entity (#915)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/915
Co-authored-by: konrad <konrad@kola-entertainments.de>
Co-committed-by: konrad <konrad@kola-entertainments.de>

pkg/db/fixtures/tasks.yml

@@ -8,7 +8,6 @@
   created: 2018-12-01 01:12:04
   updated: 2018-12-01 01:12:04
   bucket_id: 1
-  is_favorite: true
 - id: 2
   title: 'task #2 done'
   done: true
@@ -141,7 +140,6 @@
   list_id: 6
   index: 1
   bucket_id: 6
-  is_favorite: true
   created: 2018-12-01 01:12:04
   updated: 2018-12-01 01:12:04
 - id: 16
@@ -317,7 +315,6 @@
   list_id: 20
   index: 20
   bucket_id: 5
-  is_favorite: true
   created: 2018-12-01 01:12:04
   updated: 2018-12-01 01:12:04
 - id: 35